uboot-phytec

kbuild: move "checkthumb" to ARM archprepare

"checkthumb" makes sense only for ARM architecture.
Move it to arch/arm/config.mk.

To make sure gcc supports THUMB mode before beginning build,
run "checkthumb" during "archprepare".

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