selftests/sigaltstack: Fix ppc64 GCC build

selftests/sigaltstack: Fix ppc64 GCC build

Building the sigaltstack test with GCC on 64-bit powerpc errors with:

  gcc -Wall     sas.c  -o /home/michael/linux/.build/kselftest/sigaltstack/sas
  In file included from sas.c:23:
  current_stack_pointer.h:22:2: error: #error "implement current_stack_pointer equivalent"
     22 | #error "implement current_stack_pointer equivalent"
        |  ^~~~~
  sas.c: In function ‘my_usr1’:
  sas.c:50:13: error: ‘sp’ undeclared (first use in this function); did you mean ‘p’?
     50 |         if (sp < (unsigned long)sstack ||
        |             ^~

This happens because GCC doesn't define __ppc__ for 64-bit builds, only
32-bit builds. Instead use __powerpc__ to detect powerpc builds, which
is defined by clang and GCC for 64-bit and 32-bit builds.

Fixes: 05107edc9101 ("selftests: sigaltstack: fix -Wuninitialized")
Cc: stable@vger.kernel.org # v6.3+
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240520062647.688667-1-mpe@ellerman.id.au