s390/signal: remove sigreturn on stack

{rt_}sigreturn is now called from the vdso, so we no longer
need the svc on the stack, and therefore no hack to support that
mechanism on machines with non-executable stack.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>