Fix build for OS X.

This will allow the command to build and be go gettable, although on OS
X only for now.

In the future, I can use build tags to allow it to be go gettable on
other environments.

Author: dmitshur

cmd/go/zdefaultcc.go

@@ -0,0 +1,6 @@
+// auto generated by go tool dist
+
+package main
+
+const defaultCC = `clang`
+const defaultCXX = `clang++`