Add mkspecs for android cross compilation.

Add mkspecs for android cross compilation.

Using these requires setting two environment variables, e.g, for me:

export ANDROID_NDK_ROOT=/Users/burchr/android-ndk-r7c
export ANDROID_NDK_HOST=darwin-x86
./configure -opensource -confirm-license -xplatform unsupported/linux-android-armeabi-v7a-g++ -nomake examples -nomake demos -nomake tests -v

These mkspecs are somewhat based on the work of the Necessitas crew, kudos to
them for their work in getting the NDK integration into qmake.

Change-Id: I591e423ed8dc70616009f681c81890c696110e62
Reviewed-by: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>