cortex-m-rt: Remove LR push, to ensure the stack is 8-byte aligned.

This was causing incorrect execution of code optimized with the assumption the stack is 8-byte aligned.

Author: Dirbaio

cortex-m-rt/src/lib.rs

@@ -572,12 +572,6 @@ cfg_global_asm! {
      dsb
      isb",
 
-    // Push `lr` to the stack for debuggers, to prevent them unwinding past Reset.
-    // See https://sourceware.org/binutils/docs/as/CFI-directives.html.
-    ".cfi_def_cfa sp, 0
-     push {{lr}}
-     .cfi_offset lr, 0",
-
     // Jump to user main function.
     // `bl` is used for the extended range, but the user main function should not return,
     // so trap on any unexpected return.