[dev.power64] runtime: add power64/power64le as supported arch to gcinfo_test.go

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/122560043

Committer: Russ Cox <rsc@golang.org>
[dev.power64] syscall: re-generate using cgo compiled by gc, fix test failures

LGTM=dave, rsc
R=rsc, iant, dave
CC=golang-codereviews
https://codereview.appspot.com/127320043

Committer: Russ Cox <rsc@golang.org>
[dev.power64] runtime: don't fault on runtime.procyield, just return

LGTM=dave, rsc
R=rsc, iant, dave
CC=golang-codereviews
https://codereview.appspot.com/128230044

Committer: Russ Cox <rsc@golang.org>