go.tools/dashboard/builder: always cross compile for nacl builds

If we are building, for example, linux/386 on a linux/amd64 machine we want to make sure that the whole build is done as a if this were compiled on a real linux/386 machine. In other words, we want to not do a cross compilation build.

The exception to this rule is when we are doing nacl builds. These are by definition always cross compilation, and we have support built into cmd/go to be able to handle this case.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/74230043

builder/env.go

@@ -54,11 +54,20 @@ func (b *Builder) envv() []string {
 	if *buildTool == "go" {
 		e = []string{
 			"GOOS=" + b.goos,
-			"GOHOSTOS=" + b.goos,
 			"GOARCH=" + b.goarch,
-			"GOHOSTARCH=" + b.goarch,
 			"GOROOT_FINAL=/usr/local/go",
 		}
+		if b.goos != "nacl" {
+			// If we are building, for example, linux/386 on a linux/amd64 machine we want to
+			// make sure that the whole build is done as a if this were compiled on a real
+			// linux/386 machine. In other words, we want to not do a cross compilation build.
+			// To do this we set GOHOSTOS and GOHOSTARCH to override the detection in make.bash.
+			//
+			// The exception to this rule is when we are doing nacl builds. These are by definition
+			// always cross compilation, and we have support built into cmd/go to be able to handle
+			// this case.
+			e = append(e, "GOHOSTOS="+b.goos, "GOHOSTARCH="+b.goarch)
+		}
 	}
 
 	for _, k := range extraEnv {