libunwind: Do not use __attribute__((target("gcs"))) with non-clang compilers (#138077)

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/UnwindLevel1.c

@@ -188,10 +188,11 @@ extern int __unw_step_stage2(unw_cursor_t *);
 
 #if defined(_LIBUNWIND_USE_GCS)
 // Enable the GCS target feature to permit gcspop instructions to be used.
-__attribute__((target("gcs")))
+__attribute__((target("+gcs")))
 #endif
 static _Unwind_Reason_Code
-unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *exception_object) {
+unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor,
+              _Unwind_Exception *exception_object) {
   __unw_init_local(cursor, uc);
 
   _LIBUNWIND_TRACE_UNWINDING("unwind_phase2(ex_obj=%p)",
@@ -332,12 +333,12 @@ unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *except
 
 #if defined(_LIBUNWIND_USE_GCS)
 // Enable the GCS target feature to permit gcspop instructions to be used.
-__attribute__((target("gcs")))
+__attribute__((target("+gcs")))
 #endif
 static _Unwind_Reason_Code
 unwind_phase2_forced(unw_context_t *uc, unw_cursor_t *cursor,
-                     _Unwind_Exception *exception_object,
-                     _Unwind_Stop_Fn stop, void *stop_parameter) {
+                     _Unwind_Exception *exception_object, _Unwind_Stop_Fn stop,
+                     void *stop_parameter) {
   __unw_init_local(cursor, uc);
 
   // uc is initialized by __unw_getcontext in the parent frame. The first stack
@@ -443,7 +444,6 @@ unwind_phase2_forced(unw_context_t *uc, unw_cursor_t *cursor,
   return _URC_FATAL_PHASE2_ERROR;
 }
 
-
 /// Called by __cxa_throw.  Only returns if there is a fatal error.
 _LIBUNWIND_EXPORT _Unwind_Reason_Code
 _Unwind_RaiseException(_Unwind_Exception *exception_object) {