A Redox cross build cannot run the target shader-precompile tools on the host,
so build them natively once and feed them to the cross Mesa via
-Dmesa-clc=system -Dprecomp-compiler=system. Native meson build against
llvm-native's CLEAN host LLVM/Clang (the redoxer toolchain's llvm-config emits
-I<toolchain>/include, which holds Redox target libc headers that poison host
g++), host system deps (libdrm/expat/zlib/zstd), and our host-built
SPIRV-Tools/LLVMSPIRVLib/libclc (pkg-config with absolute .pc prefixes; native
pkg-config, not the cross wrapper). Driver-agnostic tools, so iris-only (no
amdgpu-LLVM needed); they serve radeonsi/radv in the cross build too.
