asyncsafe-spin: Fix compilation error with GCC on 32-bit SPARC. Reported by Paul Eggert in <https://lists.gnu.org/archive/html/bug-gnulib/2020-11/msg00066.html>. * m4/sparcv8+.m4: New file. * modules/sparcv8+: New file. * modules/asyncsafe-spin (Depends-on): Add sparcv8+.