runtime: no need to set R9 to m for runtime.sigpanic anymore

Replaces CL 123980043 which I created on the dev.power64 branch.

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/123120043
cmd/8g: fix build

Fixes issue 8510.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/129720043