Improve the documentation about building the llvmpipe driver for Android
by providing more ready-to-use scripts to build llvm and create the
needed meson wrap file.
Note: in the meson wrap file, remove 'libLLVMTableGenGlobalISel' from
the built dependencies, as apparently is not available anymore for
Android on recent LLVM versions.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32251>