x86: Merge asm/atomic_{32,64}.h

Merge the now identical code from asm/atomic_32.h and asm/atomic_64.h
into asm/atomic.h.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
LKML-Reference: <1262883215-4034-4-git-send-email-brgerst@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>