Fix GCC builds with Debug build type

When compiling with -DCMAKE_BUILD_TYPE=Debug, GCC would (correctly) fail to
compile intrinsics in stb and host1x due to lack of optimizations.

Sadly, the compilation error given is bogus and Clang completing the builds
without issues does raise some eyebrows.

Therefore, force optimizations for the offending files under GCC when
creating Debug builds.

Signed-off-by: voidanix <voidanix@keyedlimepie.org>