hurd: save xstate during signal handling

hurd: save xstate during signal handling

* hurd/Makefile: add new tests
* hurd/test-sig-rpc-interrupted.c: check xstate save and restore in
  the case where a signal is delivered to a thread which is waiting
  for an rpc. This test implements the rpc interruption protocol used
  by the hurd servers. It was so far passing on Debian thanks to the
  local-intr-msg-clobber.diff patch, which is now obsolete.
* hurd/test-sig-xstate.c: check xstate save and restore in the case
  where a signal is delivered to a running thread, making sure that
  the xstate is modified in the signal handler.
* hurd/test-xstate.h: add helpers to test xstate
* sysdeps/mach/hurd/i386/bits/sigcontext.h: add xstate to the
  sigcontext structure.
+ sysdeps/mach/hurd/i386/sigreturn.c: restore xstate from the saved
  context
* sysdeps/mach/hurd/x86/trampoline.c: save xstate if
  supported. Otherwise we fall back to the previous behaviour of
  ignoring xstate.
* sysdeps/mach/hurd/x86_64/bits/sigcontext.h: add xstate to the
  sigcontext structure.
* sysdeps/mach/hurd/x86_64/sigreturn.c: restore xstate from the saved
  context

Signed-off-by: Luca Dariz <luca@orpolo.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-ID: <20250319171118.142163-1-luca@orpolo.org>