microblaze: wire up sigaltstack

microblaze: wire up sigaltstack

sigaltstack is wired to sys_ni_syscall - microblaze is the only
architecture without it - although the microblaze signal delivery code
fully supports the alternate signal stack: get_sigframe() picks the
stack via sigsp(), setup_rt_frame() saves it with __save_altstack() and
sys_rt_sigreturn() calls restore_altstack().

Wire it up to sys_sigaltstack.

Tested on qemu petalogix-s3adsp1800 (microblazeel) with the uClibc-ng
test suite: the five sigaltstack-dependent NPTL tests (tst-cancel20/21,
tst-cancelx20/21, tst-signal6) pass; before this change sigaltstack()
returned ENOSYS.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Ramin Moussavi <ramin.moussavi@yacoub.de>
Tested-by: Waldemar Brodkorb <wbx@openadk.org>
Link: https://patch.msgid.link/20260821151809.1233057-2-ramin.moussavi@yacoub.de
Signed-off-by: Michal Simek <michal.simek@amd.com>