Android: rework LLVM build support

Android: rework LLVM build support

Currently, building with "mmma external/mesa3d" which builds all targets
and dependencies is broken for targets that require LLVM. This is due to
the build settings depending on MESA_ENABLE_LLVM. Instead of using a
conditional in the global Android.common.mk, make all the components that
need LLVM explicitly include the necessary build settings.

GALLIVM_CPP_SOURCES doesn't exist anymore, so remove that as well.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>