Add "memory" clobbers to the x86 inline asm of strncmp and friends

They don't actually clobber memory, but gcc doesn't even know they
_read_ memory, so can apparently re-order memory accesses around them.

Which obviously does the wrong thing if the memory access happens to
change the memory that the compare function is accessing..

Verified to fix a strange boot problem by Jens Axboe.
Signed-off-by: Chris Wright <chrisw@osdl.org>