This PR allows building using the SwiftWasm toolchain from https://swiftwasm.org.
As of right now, macro plugins do not build correctly for WASM, so swift-testing
is not usable, but it is buildable!

 ## Testing

A workaround is provided on the [jgrynspan/wasi-workaround](https://github.com/apple/swift-testing/tree/jgrynspan/wasi-workaround)
branch in order to facilitate testing. Check out that branch, then set the
environment variable `SWT_COMPILER_PLUGIN_ENABLED` to `0` when building:

```sh
SWT_COMPILER_PLUGIN_ENABLED=0 swift build --triple wasm32-unknown-wasi
```

The macros target should build except for its main function (which will emit a
warning about being unsupported) as should the primary library target. It is not
possible to build the testing library's tests yet.