cmd/compile: pass process env to 'go tool compile' in compileToAsm

In particular, this stops the test failing when GOROOT and GOROOT_FINAL are
different.

Change-Id: Ibf6cc0a173f1d965ee8aa31eee2698b223f1ceec
Reviewed-on: https://go-review.googlesource.com/23620
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -61,7 +61,7 @@ func compileToAsm(dir, arch, pkg string) string {
 
 	var stdout, stderr bytes.Buffer
 	cmd := exec.Command("go", "tool", "compile", "-S", "-o", filepath.Join(dir, "out.o"), src)
-	cmd.Env = append(cmd.Env, "GOARCH="+arch)
+	cmd.Env = append([]string{"GOARCH=" + arch}, os.Environ()...)
 	cmd.Stdout = &stdout
 	cmd.Stderr = &stderr
 	if err := cmd.Run(); err != nil {