Support Android x86 ABI

This allow to build `libjingle_peerconnection_so.so` for x86 ABI, like:

```shell
get_webrtc # seperated from build_apprtc

export WEBRTC_ARCH=x86
build_apprtc
```

It also simplify the build function by introducing `$WEBRTC_DEBUG`.
For more information, please see the updated README.