cmd/compile: use path.Join in importfile

Change-Id: Ib413b0cb16405965455d7764a8c4a22bf431389b
Reviewed-on: https://go-review.googlesource.com/19850
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

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

@@ -729,11 +729,7 @@ func importfile(f *Val, indent []byte) {
 		if localimport != "" {
 			prefix = localimport
 		}
-		cleanbuf := prefix
-		cleanbuf += "/"
-		cleanbuf += path_
-		cleanbuf = path.Clean(cleanbuf)
-		path_ = cleanbuf
+		path_ = path.Join(prefix, path_)
 
 		if isbadimport(path_) {
 			return