Build for Android from Windows, work in progress.

Build for Android from Windows, work in progress.

Pretty vanilla stuff here, mostly just making the gcc-like toolchain Windows friendly.

I was having trouble getting rm -r {{output}} && $ar rcs {{output}} @$rspfile to work without deleting my ar.exe, so I chickened out the usual way by adding gn/ar.py.

I've also updated bin/droid to work with Git Bash on Windows.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3751

Change-Id: I04c34ccc91e6a291c11ac4e7a7a0ffe41d879fe6
Reviewed-on: https://skia-review.googlesource.com/3751
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>