009e302df6
Bundled LLVM is never built when llvm-config is specified in
config.toml, ie PORT_LLVM, and base system LLVM doesn't support
wasm32-unknown-unknown triple. Restore the previous behaviour of
using clang from the LLVM port when PORT_LLVM is enabled. Also
really fix the ${LLVM_VERSION} logic, as the variable is from
USES=llvm.