safe_math.h: Check if __GNUC__ is defined

safe_math.h: Check if __GNUC__ is defined

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=4880

Facing the below issue after openssl is upgraded

Edk2\CryptoPkg\Library\OpensslLib\openssl\include\internal/safe_math.h(19):
warning C4668: '__GNUC__' is not defined as a preprocessor macro, replacing
with '0' for '#if/#elif'

CLA: trivial

Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Kalavakolanu Hema Anmisha <hema.anmisha.kalavakolanu@intel.com>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26204)