Ported from i386/Linus

Still won't handle other TF changing instructions like IRET or LAHF.

Prefix handling must be double checked...
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>