Fix ARMv6 fiber asm: gate Cortex-M0 branch on __ARM_ARCH_6M__

The Boost.Context 1.91.0 sync (GH-22584) pulled in boostorg/context#325,
"fix cortex-m0 support to aapcs_elf_gas target", which adds a fallback
branch to make_fcontext/jump_fcontext for the Thumb-1 encoding limits of
ARMv6-M. The branch is correct for Cortex-M0 but is gated on
__ARM_ARCH >= 7, which is also false for classic ARMv6 in ARM state
(Alpine armhf, Raspberry Pi 1/Zero) -- targets that can encode every
instruction in the >= 7 branch and need none of the workaround.

Two consequences. make_fcontext took `ldr a2, =finish', materialising the
absolute address of finish through a literal pool in .text and emitting
R_ARM_ABS32 against .text. In a PIE link that becomes an R_ARM_RELATIVE
dynamic relocation inside the read-only text segment, tagging the binary
DT_TEXTREL. musl supports DT_TEXTREL only for DSOs it maps itself; the
main executable is kernel-mapped and handled by kernel_mapped_dso(),
which never inspects it, so ldso writes to a read-only page and dies with
SIGSEGV in do_relocs() before main(). Every SAPI, every invocation, no
output. glibc masks this by mprotecting the segment writable, which is
why only musl builds noticed.

Separately, jump_fcontext -- the hot path on every fiber switch -- grew
from 13 to 43 instructions on all ARMv6 builds, glibc included.

Gate on __ARM_ARCH_6M__ instead, exactly the target #325 names. The
result emits byte-identical .text to 8.6.0alpha1 on every ARM target
except Cortex-M0, where it emits byte-identical .text to current master:
each target gets back the code it had before the regression, and M0 keeps
what #325 added. armv7 and arm64 are unaffected either way.

Zend/asm is vendored and verified by the Verify Bundled Files workflow,
so carry the same diff in .github/scripts/download-bundled and re-apply
it from the sync script, as uriparser already does.

Author: andypost

.github/scripts/download-bundled/boost-context.arm.patch

@@ -0,0 +1,71 @@
+diff --git a/Zend/asm/jump_arm_aapcs_elf_gas.S b/Zend/asm/jump_arm_aapcs_elf_gas.S
+index 4415a541fd3..647ae9f45ad 100644
+--- a/Zend/asm/jump_arm_aapcs_elf_gas.S
++++ b/Zend/asm/jump_arm_aapcs_elf_gas.S
+@@ -50,7 +50,7 @@ jump_fcontext:
+     push {lr}
+     @ save hidden,V1-V8,LR
+ 
+-    #if __ARM_ARCH >= 7
++    #if !defined(__ARM_ARCH_6M__)
+ 
+     push {a1,v1-v8,lr}
+ 
+@@ -102,7 +102,7 @@ jump_fcontext:
+     add  sp, sp, #64
+ 
+     @ restore hidden,V1-V8,LR
+-    #if __ARM_ARCH >= 7
++    #if !defined(__ARM_ARCH_6M__)
+     pop {a4,v1-v8,lr}
+     #else
+ 
+diff --git a/Zend/asm/make_arm_aapcs_elf_gas.S b/Zend/asm/make_arm_aapcs_elf_gas.S
+index 2367c75a6a7..b5cf0c8fa09 100644
+--- a/Zend/asm/make_arm_aapcs_elf_gas.S
++++ b/Zend/asm/make_arm_aapcs_elf_gas.S
+@@ -48,7 +48,7 @@
+ make_fcontext:
+     @ shift address in A1 to lower 16 byte boundary
+ 
+-    #if __ARM_ARCH >= 7
++    #if !defined(__ARM_ARCH_6M__)
+     bic  a1, a1, #15
+     #else
+     lsrs	a1, a1, #4
+@@ -56,7 +56,7 @@ make_fcontext:
+     #endif
+ 
+     @ reserve space for context-data on context-stack
+-    #if __ARM_ARCH >= 7
++    #if !defined(__ARM_ARCH_6M__)
+     sub  a1, a1, #124
+     #else
+     subs  a1, #124
+@@ -66,7 +66,7 @@ make_fcontext:
+     str  a3, [a1, #104]
+ 
+     @ compute address of returned transfer_t
+-    #if __ARM_ARCH >= 7
++    #if !defined(__ARM_ARCH_6M__)
+     add  a2, a1, #108
+     #else
+     mov  a2, a1
+@@ -77,7 +77,7 @@ make_fcontext:
+ 
+ 
+     @ compute abs address of label finish
+-    #if __ARM_ARCH >= 7
++    #if !defined(__ARM_ARCH_6M__)
+     adr  a2, finish
+     #else
+     ldr  a2, =finish
+@@ -93,7 +93,7 @@ make_fcontext:
+ 
+ finish:
+     @ exit code is zero
+-    #if __ARM_ARCH >=7
++    #if !defined(__ARM_ARCH_6M__)
+     mov  a1, #0
+     #else
+     movs	r3, #0

.github/scripts/download-bundled/boost-context.sh

@@ -41,3 +41,9 @@ mv make_x86_64_ms_pe_gas.asm make_x86_64_ms_pe_gas.S
 # add extra files
 git restore LICENSE
 git restore save_xmm_x86_64_ms_masm.asm # added in GH-18352, not an upstream boost.context file
+
+# patch customized files
+# gates the Cortex-M0 (ARMv6-M) fallback in the ARM fcontext assembly on
+# __ARM_ARCH_6M__ instead of __ARM_ARCH < 7, which also matched classic ARMv6
+# and emitted a DT_TEXTREL there; drop once fixed upstream in boost.context
+git apply -v ../../.github/scripts/download-bundled/boost-context.arm.patch

Zend/asm/jump_arm_aapcs_elf_gas.S

@@ -50,7 +50,7 @@ jump_fcontext:
     push {lr}
     @ save hidden,V1-V8,LR
 
-    #if __ARM_ARCH >= 7
+    #if !defined(__ARM_ARCH_6M__)
 
     push {a1,v1-v8,lr}
 
@@ -102,7 +102,7 @@ jump_fcontext:
     add  sp, sp, #64
 
     @ restore hidden,V1-V8,LR
-    #if __ARM_ARCH >= 7
+    #if !defined(__ARM_ARCH_6M__)
     pop {a4,v1-v8,lr}
     #else
 

Zend/asm/make_arm_aapcs_elf_gas.S

@@ -48,15 +48,15 @@
 make_fcontext:
     @ shift address in A1 to lower 16 byte boundary
 
-    #if __ARM_ARCH >= 7
+    #if !defined(__ARM_ARCH_6M__)
     bic  a1, a1, #15
     #else
     lsrs	a1, a1, #4
     lsls	a1, a1, #4
     #endif
 
     @ reserve space for context-data on context-stack
-    #if __ARM_ARCH >= 7
+    #if !defined(__ARM_ARCH_6M__)
     sub  a1, a1, #124
     #else
     subs  a1, #124
@@ -66,7 +66,7 @@ make_fcontext:
     str  a3, [a1, #104]
 
     @ compute address of returned transfer_t
-    #if __ARM_ARCH >= 7
+    #if !defined(__ARM_ARCH_6M__)
     add  a2, a1, #108
     #else
     mov  a2, a1
@@ -77,7 +77,7 @@ make_fcontext:
 
 
     @ compute abs address of label finish
-    #if __ARM_ARCH >= 7
+    #if !defined(__ARM_ARCH_6M__)
     adr  a2, finish
     #else
     ldr  a2, =finish
@@ -93,7 +93,7 @@ make_fcontext:
 
 finish:
     @ exit code is zero
-    #if __ARM_ARCH >=7
+    #if !defined(__ARM_ARCH_6M__)
     mov  a1, #0
     #else
     movs	r3, #0