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)