Add a `--builddir` option to `Tools/wasm/wasi`

[brettcannon]

This will let one do builds entirely outside of one's checkout instead of forcing one to use ./cross-build/.

[brettcannon]

OK, making this happen is a lot more work than I was anticipating (e.g. having to map both Lib/ and the build directory into the WASI guest). Since no one has actually asked for this I'm going to skip doing it.