Add AMDGPU to LLVM_TARGETS_TO_BUILD.
Remove explicit CMAKE_CXX_COMPILER from libclc to dodge regression from
newer CMake. The potential range seems like (3.19.7, 3.20.1].
Add -Dc_std=c17 to support static_assert.
Add -Dcpp_std=vc++latest to support designated initializers.
Add -Dvulkan-drivers=amd for RADV.
Add -Dlibelf:warning_level=1 because of warnings as error.
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6162>