Make use of LLVM tools

Android: Prepare for the worst, when the NDK will remove all GNU tools

In NDKr18 Google removed GCC, most probably the massacre will not end
there and they will remove all GNU tools, so we need to start using LLVM
ones.
This patch still keeps using the GNU tools for the moment, but the users
can overide it by settings:

ANDROID_USE_LLVM = true

in thier qmake project files.