sh: Plug in support for ARCH=sh64 using sh SRCARCH.

This adds in support for building with ARCH=sh64 using the sh SRCARCH.
This tidies up the randconfig generation somewhat to make sure that we
don't end up with impossible configurations, and without having to rely
on things like KCONFIG_ALLCONFIG to detect the proper CPU support subset.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>