[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>

src/cmd/compile/internal/gc/go.go

@@ -293,4 +293,8 @@ var (
 	typedmemmove,
 	typedmemclr,
 	Udiv *obj.LSym
+
+	// GO386=387
+	ControlWord64trunc,
+	ControlWord32 *obj.LSym
 )

src/cmd/compile/internal/gc/ssa.go

@@ -92,6 +92,10 @@ func initssaconfig() {
 	typedmemmove = Sysfunc("typedmemmove")
 	typedmemclr = Sysfunc("typedmemclr")
 	Udiv = Sysfunc("udiv")
+
+	// GO386=387 runtime functions
+	ControlWord64trunc = Sysfunc("controlWord64trunc")
+	ControlWord32 = Sysfunc("controlWord32")
 }
 
 // buildssa builds an SSA function for fn.

src/cmd/compile/internal/x86/387.go

@@ -120,7 +120,7 @@ func ssaGenValue387(s *gc.SSAGenState, v *ssa.Value) {
 			p = s.Prog(x86.AFLDCW)
 			p.From.Type = obj.TYPE_MEM
 			p.From.Name = obj.NAME_EXTERN
-			p.From.Sym = gc.Sysfunc("controlWord32")
+			p.From.Sym = gc.ControlWord32
 		}
 
 		var op obj.As
@@ -210,7 +210,7 @@ func ssaGenValue387(s *gc.SSAGenState, v *ssa.Value) {
 		p = s.Prog(x86.AFLDCW)
 		p.From.Type = obj.TYPE_MEM
 		p.From.Name = obj.NAME_EXTERN
-		p.From.Sym = gc.Sysfunc("controlWord64trunc")
+		p.From.Sym = gc.ControlWord64trunc
 
 		// Now do the conversion.
 		p = s.Prog(x86.AFMOVLP)