alpha: Fix signal thunk unwind info

2013-11-16  Richard Henderson  <rth@redhat.com>

	* sysdeps/unix/sysv/linux/alpha/rt_sigaction.S: Include a nop
	before each signal thunk.

URL: https://bugs.gentoo.org/480740
URL: https://sourceware.org/ml/libc-ports/2013-11/msg00000.html

(cherry picked from commit 027e32bd)