Fix a braino with r259730: we cannot currently use CFLAGS.gcc or
CFLAGS.clang in sys/conf/Makefile.arm, since the main kernel build does
not use <bsd.sys.mk>. So revert that particular change for now.

Pointy hat to: me
Noticed by: zbb
MFC after: 3 days
X-MFC-With: r259730