ppc_cpudetect.c:15:10: fatal error: asm/cputable.h: No such file or directory

PPC check is broken on macOS:
```
/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_multimedia_aom/aom/work/aom-3.5.0/aom_ports/ppc_cpudetect.c:15:10: fatal error: asm/cputable.h: No such file or directory
   15 | #include <asm/cputable.h>
      |          ^~~~~~~~~~~~~~~~
compilation terminated.
```