[libunwind] Be more careful about enabling GCS (#101973)

We need both GCS to be enabled by the compiler (which we do by checking
if __ARM_FEATURE_GCS_DEFAULT is defined) and for arm_acle.h to define
the GCS intrinsics. Check the latter by checking if _CHKFEAT_GCS is
defined.

Author: john-brawn-arm

libunwind/src/cet_unwind.h

@@ -39,9 +39,13 @@
 // need to guard any use of GCS instructions with __chkfeat though, as GCS may
 // not be enabled.
 #if defined(_LIBUNWIND_TARGET_AARCH64) && defined(__ARM_FEATURE_GCS_DEFAULT)
-#define _LIBUNWIND_USE_GCS 1
 #include <arm_acle.h>
 
+// We can only use GCS if arm_acle.h defines the GCS intrinsics.
+#ifdef _CHKFEAT_GCS
+#define _LIBUNWIND_USE_GCS 1
+#endif
+
 #define _LIBUNWIND_POP_CET_SSP(x)                                              \
   do {                                                                         \
     if (__chkfeat(_CHKFEAT_GCS)) {                                             \