[PATCH] x86_64: Save/restore CS in 64bit signal handlers and force __USER_CS for CS

This allows to run 64bit signal handlers in 64bit processes that run small
code snippets in compat mode.

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