package/mesa3d: enable llvm support

package/mesa3d: enable llvm support

This patch provides LLVM support for Mesa3D, enabling llvmpipe
software rasterizer if Gallium swrast is selected.

In case Gallium r600 is selected, llvm AMDGPU backend is built.

Having llvm installed also enables radeonsi Gallium driver, but
it is not provided with this patch as it hasn't been tested.

It uses llvm-config (host variant) installed in STAGING_DIR/usr/bin
to get LLVM libraries. Assuming that LLVM version 5.0.1 is installed,
llvm-config --libs will output -lLLVM-5.0.

Signed-off-by: Valentin Korenblit <valentin.korenblit@smile.fr>
Tested-by: Jérôme Oufella <jerome.oufella@savoirfairelinux.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>