runtime: mark asmcgocall<>(SB) as having no arguments

It doesn't.
Fixes 386 build.

While we're here, mark runtime.asmcgocall as GO_ARGS,
so that it will work with stack copying. I don't think anything
that uses it can lead to a stack copy, but better safe than sorry.
Certainly the runtime.asmcgocall_errno variant needs
(and already has) GO_ARGS.

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/138400043