meson: build "radv" vulkan driver for radeon hardware

meson: build "radv" vulkan driver for radeon hardware

This builds, installs, and has been tested on a r290x (Hawaii) with the Vulkan
CTS. It dies horribly in a fire at the same point for the meson build as the
autotools build.

v2: - enable radv by default
    - add shader cache support and enforce that it's built for radv
v3: - Fix typo in meson_options (Nicholas)
    - strip trailing 'svn' from llvm version before setting the version
      preprocessor flag (Bas)
    - Check for LLVM module requirements

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>