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: I8386eaa1e05879c25c672b9c9fc693c938e9aeb6
Reviewed-on: https://go-review.googlesource.com/54090
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>
TryBot-Result: Gobot Gobot <gobot@golang.org>