(Android) restrict abi targets

This fixes a build error I get with the latest android NDK (r10) complaining about an unsupported architecture (armeabi, maybe it was retired). Also, libretro-build-android-mk.sh only builds the cores for armeabi-v7a, mips and x86 anyway so there is no point in building the app for other architectures.
Merge pull request #841 from l3iggs/patch-4

(Android) restrict abi targets