Hurd x86_64: add unwind support for signal trampoline code

Tested with some simple toy examples where an exception is thrown in the
signal handler.

libgcc/ChangeLog:
	* config/i386/gnu-unwind.h: Support unwinding x86_64 signal frames.

Signed-off-by: Flavio Cruz <flaviocruz@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>