go/build: use GCCGOTOOLDIR to find gccgo tools

Add the environment variable GCCGOTOOLDIR to permit overriding the default
    directory where tools like cgo are found when building with gccgo.
    
    This will be used by the cmd/go tests in a future CL.
    
    Reviewed-on: https://go-review.googlesource.com/45694

From-SVN: r249196

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-6d1d558109b5f5e53b31cc3325485dbb9f06d430
+a790c5fec06ced726c7f0f597186883ce61d0a45
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/cmd/go/alldocs.go

@@ -1143,6 +1143,9 @@
 // 		Defined by Git. A colon-separated list of schemes that are allowed to be used
 // 		with git fetch/clone. If set, any scheme not explicitly mentioned will be
 // 		considered insecure by 'go get'.
+//	GCCGOTOOLDIR
+//		If set, where to find gccgo tools, such as cgo.
+//		The default is based on how gccgo was configured.
 //
 //
 // Import path syntax

libgo/go/cmd/go/help.go

@@ -508,6 +508,9 @@ Special-purpose environment variables:
 		Defined by Git. A colon-separated list of schemes that are allowed to be used
 		with git fetch/clone. If set, any scheme not explicitly mentioned will be
 		considered insecure by 'go get'.
+	GCCGOTOOLDIR
+		If set, where to find gccgo tools, such as cgo.
+		The default is based on how gccgo was configured.
 	`,
 }
 

libgo/go/go/build/build.go

@@ -1549,7 +1549,7 @@ func init() {
 
 func getToolDir() string {
 	if runtime.Compiler == "gccgo" {
-		return runtime.GCCGOTOOLDIR
+		return envOr("GCCGOTOOLDIR", runtime.GCCGOTOOLDIR)
 	} else {
 		return filepath.Join(runtime.GOROOT(), "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
 	}