clover: fix TargetRegistry.h not found with llvm 14
llvm 14 moved some files from Support to MC folder, see https://github.com/llvm/llvm-project/commit/89b57061f7b769e9ea9bf6ed686e284f3e55affe
this MR uses src/gallium/frontends/clover/llvm/compat.hpp to ensure mesa sourcecode will build with all supported llvm versions.
fixes #5471 (closed)