x86: use cpu/common.c on 64 bit

Use cpu/common.c on both 64-bit and 32-bit and remove cpu/common_64.c.

We started out with this linecount:

  816  arch/x86/kernel/cpu/common_64.c
  805  arch/x86/kernel/cpu/common.c

and the resulting common.c is 1197 lines long, so there's already
424 lines of code eliminated in this phase of the unification.

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