vpmsum_crc32: Per the PPC64 ABI, v20-v31 are non-volatile register

See http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html#REG for calling convention.

In experiments under GDB, we see that after crc32_vpmsum is called that registers v20-v31 are modified. We discovered this issue after integrating the crc32_vpmsum code into our C++ code base which targeted POWER8 for code generation, see https://jira.mongodb.org/browse/SERVER-22773 for repro.