Use GCC on 32bit Linux

From Chrome 38, Chromium has started to use its own clang binaries for building by default, however its clang binaries only have 64bit build so it requires a 32bit chroot environment on a 64bit machine with special setup to build for 32bit target, see [Switch linux to use clang by default](https://code.google.com/p/chromium/issues/detail?id=360311).

In order to make building easier, we just use GCC for building on 32bit host machines.