cmd/dist: copy textflag.h for runtime/internal/* too

Change-Id: I22216df83898e560cfe6d97344f05e3678c2db99
Reviewed-on: https://go-review.googlesource.com/16872Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/dist/build.go

@@ -620,7 +620,7 @@ func install(dir string) {
 	}
 
 	// For package runtime, copy some files into the work space.
-	if dir == "runtime" {
+	if dir == "runtime" || strings.HasPrefix(dir, "runtime/internal/") {
 		xmkdirall(pathf("%s/pkg/include", goroot))
 		// For use by assembly and C files.
 		copyfile(pathf("%s/pkg/include/textflag.h", goroot),