gn build: Add gn.py wrapper script that adds --dotfile= and --root= parameters

Since people weren't enthused about moving the .gn file to the toplevel in
D56419, here's a script to make gn at least somewhat more pleasant to invoke
(useful for gn clean, gn args --list, gn desc, etc).

Differential Revision: https://reviews.llvm.org/D56565

llvm-svn: 351064