MFC r302438:
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
Approved by: re (gjb)