[dev.power64] hash/crc32: use portable castagnoli on power64
[dev.power64] hash/crc32: use portable castagnoli on power64

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/128110044

bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh