android: Enable llvmpipe when using the swrast driver

This merge request enables llvmpipe when using the swrast driver on Android.

It builds upon Wu Zhen's patches and hopes to answer all previous concerns. It is more limited in scope and takes into account the evolution of Mesa's build files for Android.

Please take it into consideration, as both Replicant and Android-x86 projects are currently maintaining their own patches for this.

Reviews and fixes are much welcomed.

Thank you for your time.

Summary:

• Add src/gallium/drivers/llvmpipe/Android.mk file
• Include drivers/llvmpipe on src/gallium/Android.mk
• Add llvmpipe reference to the gallium_drivers list on Android.mk
• Include libLLVM as a shared library when building the gallium_dri module