rustc_target: Add efiapi ABI support for LoongArch

This commit adds basic `efiapi` ABI support for LoongArch by recognizing
`extern "efiapi"` in the ABI map and inline asm clobber handling, and
mapping it to the C calling convention.

This change is intentionally submitted ahead of the full LoongArch UEFI
target support. While UEFI binaries are ultimately produced as PE images,
LoongArch UEFI applications can already be developed by building ELF
objects, applying relocation fixups, and converting them to PE in a
later step. For such workflows, having `efiapi` properly recognized by
the compiler is a prerequisite, even without a dedicated UEFI target.

Landing this ABI support early helps unblock LoongArch UEFI application
and driver development, and allows the remaining UEFI-specific pieces to
be introduced incrementally in follow-up patches.

MCP: rust-lang/compiler-team#953

Author: heiher

compiler/rustc_target/src/asm/mod.rs

@@ -997,8 +997,8 @@ impl InlineAsmClobberAbi {
                 _ => Err(&["C", "system"]),
             },
             InlineAsmArch::LoongArch32 | InlineAsmArch::LoongArch64 => match name {
-                "C" | "system" => Ok(InlineAsmClobberAbi::LoongArch),
-                _ => Err(&["C", "system"]),
+                "C" | "system" | "efiapi" => Ok(InlineAsmClobberAbi::LoongArch),
+                _ => Err(&["C", "system", "efiapi"]),
             },
             InlineAsmArch::PowerPC | InlineAsmArch::PowerPC64 => match name {
                 "C" | "system" => Ok(if target.abi == Abi::Spe {

compiler/rustc_target/src/spec/abi_map.rs

@@ -56,6 +56,7 @@ impl AbiMap {
                 ArmVer::Other
             }),
             Arch::Avr => ArchKind::Avr,
+            Arch::LoongArch32 | Arch::LoongArch64 => ArchKind::LoongArch,
             Arch::Msp430 => ArchKind::Msp430,
             Arch::Nvptx64 => ArchKind::Nvptx,
             Arch::RiscV32 | Arch::RiscV64 => ArchKind::Riscv,
@@ -108,7 +109,10 @@ impl AbiMap {
 
             (ExternAbi::EfiApi, ArchKind::Arm(..)) => CanonAbi::Arm(ArmCall::Aapcs),
             (ExternAbi::EfiApi, ArchKind::X86_64) => CanonAbi::X86(X86Call::Win64),
-            (ExternAbi::EfiApi, ArchKind::Aarch64 | ArchKind::Riscv | ArchKind::X86) => CanonAbi::C,
+            (
+                ExternAbi::EfiApi,
+                ArchKind::Aarch64 | ArchKind::LoongArch | ArchKind::Riscv | ArchKind::X86,
+            ) => CanonAbi::C,
             (ExternAbi::EfiApi, _) => return AbiMapping::Invalid,
 
             /* arm */
@@ -196,6 +200,7 @@ enum ArchKind {
     Amdgpu,
     Arm(ArmVer),
     Avr,
+    LoongArch,
     Msp430,
     Nvptx,
     Riscv,