xtensa: fix signal delivery to FDPIC process

commit 9c2cc74f upstream.

Fetch function descriptor pointed to by the signal handler pointer from
userspace on signal delivery and function pointer pointed to by the
sa_restorer on return from the signal handler.

Cc: stable@vger.kernel.org
Fixes: e3ddb8bb ("xtensa: add FDPIC and static PIE support for noMMU")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>