libunwind: Do not use __attribute__((target("gcs"))) with non-clang compilers

This attribute is unsupported in GCC, so far it worked because before GCC15
did not define this macros in _CHKFEAT_GCS in arm_acle.h [1]

With gcc15 compiler libunwind's check for this macros is succeeding and it
ends up enabling 'gcs' by using function attribute, this works with clang
but not with gcc.

We can see this in rust compiler bootstrap for aarch64/musl when system
uses gcc15, it ends up with these errors

Building libunwind.a for aarch64-poky-linux-musl
cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:191:1: error: arch extension 'gcs' should be prefixed by '+'
cargo:warning=  191 | unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *exception_object) {
cargo:warning=      | ^~~~~~~~~~~~~
cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:337:22: error: arch extension 'gcs' should be prefixed by '+'
cargo:warning=  337 |                      _Unwind_Stop_Fn stop, void *stop_parameter) {
cargo:warning=      |                      ^~~~~~~~~~~~~~~

[1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=5a6af707f0af

Signed-off-by: Khem Raj <[email protected]>

Author: kraj

libunwind/src/shadow_stack_unwind.h

@@ -42,7 +42,8 @@
 #include <arm_acle.h>
 
 // We can only use GCS if arm_acle.h defines the GCS intrinsics.
-#ifdef _CHKFEAT_GCS
+// it uses attribute 'gcs' in source which does not work with gcc
+#if defined(_CHKFEAT_GCS) && defined(__clang__)
 #define _LIBUNWIND_USE_GCS 1
 #endif