Fix a pretty bad bug that caused sometimes signals on x86-64
to be restarted like system calls. This corrupted the RIP and
in general caused undesirable effects.

The problem happens because orig_rax is unsigned on x86-64,
but it originally was signed when the signal code was written.
And the if (orig_rax >= 0) ended up always true.
And gcc didn't warn about this, because the warning is only in 
-Wextra. 

In 2.4 we still had a cast for it, but somehow it got dropped
in 2.5.

Credit goes to John Slice for tracking it down and Erich Boleyn
for the original fix. All blame to me. I fixed it at another
place too.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>