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

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/128110044
[dev.power64] cmd/9a: use new flag parser

Copy main from 6a. Fixes various things, but the
main thing is the use of the new flag parser.
The go command expects to be able to use -trimpath
and not have it be interpreted as -t rimpath.

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/126100043
[dev.power64] reflect: add assembly stubs

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/126090043