llvm: allow building libllvm in native builds, subject to PACKAGECONFIG

llvm: allow building libllvm in native builds, subject to PACKAGECONFIG

Also, enable that, if opengl is in native DISTRO_FEATURES: this allows
mesa-native to build drivers that rely on libllvm, particularly llvmpipe,
which is a (sort of) accelerated software renderer that is the only
option on build hosts without dedicated or supported GPUs.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>