x86: Add BITS to allow simple Makefile sharing

Preperatory patch to simplify the sharing of Makefiles in
arch/x86.

Linus came up with this during a discussion about the ugliness of
ifeq($CONFIG_X86_32),y) and obj-$(CONFIG_X86_32) in the shared
Makefiles.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>