score: fix bogus restarts on sigreturn()

we *really* don't want to have restart logics hit when we are returning from
sigreturn() - random replacement of %r4 with -4 just because a signal had
been noticed from timer interrupt that came when %r4 happened to contain
-514 is not nice at all.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>