x86: allow more than 8 cpus to be used on 32-bit

X86_PC is the only remaining 'sub' architecture, so we dont need
it anymore.

This also cleans up a few spurious references to X86_PC in the
driver space - those certainly should be X86.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>