added android x86 support to NDK toolchain

Hi,

I have added support for android x86 target.
Basically added the x86 prebuilts for 3rd party libraries that are used and fixed code and toolchain so it correctly compiles an additional libepub3.so library inside the generated libs/x86 folder.
I've compiled it with NDK r10b and tested it with 'SDKLauncher-Android' and  'Readium_SDK_Test_Android' on a x86 Android device without encountering any issue.