Fix compilation warning on aarch64-linux (#56480)

This fixes the warning:
```
/cache/build/default-aws-aarch64-ci-1-3/julialang/julia-master/src/stackwalk.c: In function 'jl_simulate_longjmp':
/cache/build/default-aws-aarch64-ci-1-3/julialang/julia-master/src/stackwalk.c:995:22: warning: initialization of 'mcontext_t *' {aka 'struct sigcontext *'} from incompatible pointer type 'struct unw_sigcontext *' [-Wincompatible-pointer-types]
  995 |     mcontext_t *mc = &c->uc_mcontext;
      |                      ^
```

This is the last remaining warning during compilation on aarch64-linux.

(cherry picked from commit 8593792)

Author: giordano

src/stackwalk.c

@@ -919,7 +919,13 @@ static void jl_rec_backtrace(jl_task_t *t) JL_NOTSAFEPOINT
         memset(&c, 0, sizeof(c));
      #if defined(_OS_LINUX_) && defined(__GLIBC__)
         __jmp_buf *mctx = &t->ctx.ctx.uc_mcontext->__jmpbuf;
+        #if defined(_CPU_AARCH64_)
+        // Only on aarch64-linux libunwind uses a different struct than system's one:
+        // <https://github.com/libunwind/libunwind/blob/e63e024b72d35d4404018fde1a546fde976da5c5/include/libunwind-aarch64.h#L193-L205>.
+        struct unw_sigcontext *mc = &c.uc_mcontext;
+        #else
         mcontext_t *mc = &c.uc_mcontext;
+        #endif
       #if defined(_CPU_X86_)
         // https://github.com/bminor/glibc/blame/master/sysdeps/i386/__longjmp.S
         // https://github.com/bminor/glibc/blame/master/sysdeps/i386/jmpbuf-offsets.h