x86: make 64 bit to use amd.c

arch/x86/kernel/cpu/amd.c is now 100% identical to
arch/x86/kernel/cpu/amd_64.c, so use amd.c on 64-bit too
and fix up the namespace impact.

Simplify the Kconfig glue as well.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>