xtensa: fix signal delivery to FDPIC process

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>