x86, atomic64: In selftest, distinguish x86-64 from 586+

The x86-64 implementation of the atomics is totally different from the
i586+ implementation, which makes it quite confusing to call it
"586+".  Also fix indentation, and add "i" for "i386" and "i586" as
used elsewhere in the kernel.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Luca Barbieri <luca@luca-barbieri.com>
LKML-Reference: <1267005265-27958-4-git-send-email-luca@luca-barbieri.com>