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>
diff --git a/bin/droid b/bin/droid
index b1644c6..c633181 100755
--- a/bin/droid
+++ b/bin/droid
@@ -16,6 +16,7 @@
 set -e
 set -x
 
-adb push $path /data/local/tmp/
-adb push resources /data/local/tmp/
-adb shell "cd /data/local/tmp; ./$name $args"
+adb push $path //data/local/tmp/
+adb push resources //data/local/tmp/
+adb shell "chmod +x //data/local/tmp/$name"
+adb shell "cd //data/local/tmp; ./$name $args"