Use la/jr pseudoinstructions for main function call

Replace direct jal with load address and jump register pseudoinstructions
to support main functions located beyond the ±1MB range of direct jumps.

Author: janderholm

riscv-rt/CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+### Changed
+
+- Main function no longer needs to be close to _start. A linker script may copy
+  all code to RAM and keep .init in flash/ROM.
+
 ## [v0.15.0] - 2025-06-10
 
 ### Added

riscv-rt/src/asm.rs

@@ -217,7 +217,8 @@ cfg_global_asm!(
     ld a1, 8 * 1(sp)
     ld a2, 8 * 2(sp)
     addi sp, sp, 8 * 4",
-    "jal zero, main
+    "la t0, main
+    jr t0
     .cfi_endproc",
 );