Always use the standard OSX SDK tools

Instead of whatever happens to be in PATH.

Test: port install gcc5; attempt to build host binaries
Change-Id: If4af32596edd6fd3e459c3d574ab0ff495f236ff
diff --git a/cc/compiler.go b/cc/compiler.go
index dc594e3..84ee652 100644
--- a/cc/compiler.go
+++ b/cc/compiler.go
@@ -262,10 +262,7 @@
 		flags.LdFlags = config.ClangFilterUnknownCflags(flags.LdFlags)
 
 		target := "-target " + tc.ClangTriple()
-		var gccPrefix string
-		if !ctx.Darwin() {
-			gccPrefix = "-B" + filepath.Join(tc.GccRoot(), tc.GccTriple(), "bin")
-		}
+		gccPrefix := "-B" + config.ToolPath(tc)
 
 		flags.CFlags = append(flags.CFlags, target, gccPrefix)
 		flags.AsFlags = append(flags.AsFlags, target, gccPrefix)