Compile ppc.c on all powerpc machines. Fixes 32 bit powerpc build.

Use consistent condition for ppc probing. Fixes 32 bit powerpc build.

According to src/arch/CMakeLists.txt the ppc probing code is compiled when HAVE_POWER8 is true. The condition for when checking the ppc probe result in other places in the code must therfore also check if HAVE_POWER8 is defined. Otherwise compilation fails with

/usr/bin/ld: ../lib/libceph-common.so.0: undefined reference to `ceph_arch_ppc_probe'
/usr/bin/ld: ../lib/libceph-common.so.0: undefined reference to `ceph_arch_ppc_crc32'
/usr/bin/ld: ../lib/libceph-common.so.0: undefined reference to `ceph_crc32c_ppc'
collect2: error: ld returned 1 exit status

Reported upstream: https://github.com/ceph/ceph/pull/42928