Do not use -fformat-extensions with non-base versions of gcc

Ports versions of gcc do not have -fformat-extensions support.

This unbreaks compiling the kernel/modules with non-base gcc (4.8,
5.0, etc) if MK_FORMAT_EXTENSIONS=yes (the default).

Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D7150
Reviewed by: bdrewery
Sponsored by: EMC / Isilon Storage Division