[release-branch.go1.5] runtime: make asmcgocall work without a g

Solaris needs to make system calls without a g,
and Solaris uses asmcgocall to make system calls.
I know, I know.

I hope this makes CL 16915, fixing #12277, work on Solaris.

Change-Id: If988dfd37f418b302da9c7096f598e5113ecea87
Reviewed-on: https://go-review.googlesource.com/17072


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/17129