kbuild: move "checkgcc4" to PowerPC archprepare

"checkgcc4" is used only for PowerPC.
Move it to arch/powerpc/config.mk.

To make sure gcc is new enough before beginning build,
run "checkgcc4" during "archprepare".

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>