[bsd.hardening.mk] Use '-ftrivial-auto-var-init=zero' with GCC.
Stack Auto-Zero-Initialzation happen soon in GCC, See:
https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=a25e0b5e6ac8a77a71c229e0a7b744603365b0e9
We will be able to build ports that use 'USE_GCC' with the "-ftrivial-auto-var-init=zero" argument (See: bsd.hardening.mk#L361)