This patch adds "_multithreaded" as a valid `_runtime` argument and
sets it when the target is `wasm32-unknown-wasi-threads` or other
non-none OS targets.
This adds the initial conditional compilation support for the WASM32
"architecture" assuming that WASI is used as the "OS". Support for
baremetal targets in Swift needs more work still, but this gives enough
infrastructure to start playing with WASM.