cmd/compile: clarify where mkbuiltin.go input files are found

Just a comment change reflecting that the files were moved to the
builtin directory when the compiled was converted from C to Go.

Change-Id: I65e5340c09221684e40174feadfb69f738a9044a
Reviewed-on: https://go-review.googlesource.com/16089Reviewed-by: Minux Ma <minux@golang.org>

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

@@ -4,8 +4,9 @@
 
 // +build ignore
 
-// Generate builtin.go from $* (runtime.go and unsafe.go).
-// Run this after changing runtime.go and unsafe.go
+// Generate builtin.go from builtin/runtime.go and builtin/unsafe.go
+// (passed as arguments on the command line by a go:generate comment).
+// Run this after changing builtin/runtime.go and builtin/unsafe.go
 // or after changing the export metadata format in the compiler.
 // Either way, you need to have a working compiler binary first.
 package main