Revert r302403

lang/gcc{48,49,5} lacks -fformat-extensions support (causing build errors, which
is what prompted r302403 to be committed). devel/amd64-gcc on the other hand
(which is used by Jenkins), has the support.

This fixes the Jenkins failure emails due to excessive warnings being produced
with "make buildkernel".

Discussed with: lwhsu
Reported by: Jenkins (FreeBSD_HEAD_amd64_gcc job)
Sponsored by: EMC / Isilon Storage Division