[klibc] alpha: Pass restorer to rt_sigaction() and disable executable stack

[klibc] alpha: Pass restorer to rt_sigaction() and disable executable stack

Origin: https://git.kernel.org/pub/scm/libs/klibc/klibc.git/commit/?id=570ed1e207cbe38ed487c722f8ac7db68e664a94

alpha does not support the SA_RESTORER flag, but allows specifiying a
restorer callback as an additional parameter to rt_sigaction().  We
should do this to avoid needing an executable stack.

* Force the SA_SIGINFO flag on for all signal handlers, so that we can
  always return from them with rt_sigreturn
* Define a __sigreturn() routine that calls rt_sigreturn
* Pass that routine as the last argument to rt_sigaction()
* Set KLIBCEXECSTACK=n

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Name 0004-klibc-alpha-Pass-restorer-to-rt_sigaction-and-disabl.patch