[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.

GitOrigin-RevId: c649194a71b47431f2eb2e041435d564e3b51072