[ref model] Remove incompatible hardened flags

-fcf-protection=full is not compatible with aarch64 targets and
-D_FORTIFY_SOURCE=3 was introduced in GCC 12, while we need to support
older versions too.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I52f70bbbbf532672aae26b670b6f1ee8d0ebc90c