[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.