Rename Android target triple

[brson]

Currently rustc uses arm-unknown-android but this doesn't seem to be the preferred triple. I think @sanxiyn mentioned a better one but I can't find it. Maybe arm-linux-androidabi.

[alexrp]

arm-linux-androideabi is the triple used by the Android NDK.

[yichoi]

You read my mind. I will do that at once

[yichoi]

I renamed arm-unknown-android to arm-linux-androideabi, however more delicate function of get_os and get_arch in driver.rs needed since str::contains cause error by order. I will update patch soon

[yichoi]

I will update doc after patch landed

[yichoi]

I've updated doc
It reflects renaming arm-unknon-linux to arm-linux-androideabi