[dev.power64] runtime: fix newstackcall

moreframesize is a uint32, not a uint64.

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/124310043
[dev.power64] runtime: fix build

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/128180044

Committer: Russ Cox <rsc@golang.org>