i386: Make CMPXCHG64 only dependent on PAE

It is only used for PAE kernels in set_64bit.

The problem is that due to a old Windows bug many CPUs need magic MSRs
to enable CMPXCHG64, and we can't do that nicely early enough before
it is potentially used.

But since we only need it in PAE kernels so only force the checking
for CMPXCHG65 with PAE.

This fixes a boot failure on Transmeta Crusoe

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>