[release-branch.go1.9] cmd/compile: remove gc.Sysfunc calls from 387 backend

[This is a cherry-pick of CL 54090 to the 1.9 release branch.]

gc.Sysfunc must not be called concurrently.
We set up runtime routines used by the backend
prior to doing any backend compilation.
I missed the 387 ones; fix that.

Sysfunc should have been unexported during 1.9.
I will rectify that in a subsequent CL.

Fixes #21352

Change-Id: I485bb1867b46d8e5cf64bc820b8963576dc16174
Reviewed-on: https://go-review.googlesource.com/55970


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>