runtime: allow to call stdcall from Go on windows
I've started with just one function with 8 arguments,
but stdcall is called from nosplit functions
and 8 args overflow nosplit area.

LGTM=aram, alex.brainman
R=golang-codereviews, aram, alex.brainman, dave
CC=golang-codereviews, iant, khr, rsc
https://codereview.appspot.com/135090043