x86: Split atomic64_t functions into seperate headers

Split atomic64_t functions out into separate headers, since they will
not be practical to merge between 32 and 64 bits.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
LKML-Reference: <1262883215-4034-2-git-send-email-brgerst@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>