cmd/compile: look up more runtime symbols before SSA begins

This avoids concurrent runtime package lookups.

Updates #15756

Change-Id: I9e2cbd042aba44923f0d03e6ca5b4eb60fa9e7ea
Reviewed-on: https://go-review.googlesource.com/40853
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -265,5 +265,10 @@ var (
 	assertE2I,
 	assertE2I2,
 	assertI2I,
-	assertI2I2 *obj.LSym
+	assertI2I2,
+	goschedguarded,
+	writeBarrier,
+	writebarrierptr,
+	typedmemmove,
+	typedmemclr *obj.LSym
 )

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

@@ -85,6 +85,11 @@ func initssaconfig() {
 	assertE2I2 = Sysfunc("assertE2I2")
 	assertI2I = Sysfunc("assertI2I")
 	assertI2I2 = Sysfunc("assertI2I2")
+	goschedguarded = Sysfunc("goschedguarded")
+	writeBarrier = Sysfunc("writeBarrier")
+	writebarrierptr = Sysfunc("writebarrierptr")
+	typedmemmove = Sysfunc("typedmemmove")
+	typedmemclr = Sysfunc("typedmemclr")
 }
 
 // buildssa builds an SSA function.
@@ -4942,7 +4947,20 @@ func (e *ssafn) UseWriteBarrier() bool {
 }
 
 func (e *ssafn) Syslook(name string) *obj.LSym {
-	return Linksym(syslook(name).Sym)
+	switch name {
+	case "goschedguarded":
+		return goschedguarded
+	case "writeBarrier":
+		return writeBarrier
+	case "writebarrierptr":
+		return writebarrierptr
+	case "typedmemmove":
+		return typedmemmove
+	case "typedmemclr":
+		return typedmemclr
+	}
+	Fatalf("unknown Syslook func %v", name)
+	return nil
 }
 
 func (n *Node) Typ() ssa.Type {