gn: generate last_commit_position.h without git

For Git to work properly, I used fetchgit with leaveDotGit. This seems
to be causing hash to change on different systems in different times.
I've replaced generation of last_commit_position.h in tools/gen.py with
just plain nix template. "gn --version" will loose a bit (just commit
hash, without commit height in front of it), but I hope noone relies on
it.