By treating all 'architectures' containing a `-` as `-mtriple` values.
E.g., `dub --arch=wasm32-unknown-unknown-wasm` to cross-compile to
WebAssembly. This dub switch is already used when probing the compiler,
so the build platform detection works as expected.

Also add `wasm` as detected platform and use the .wasm executable file
extension (for ldc2).

This fixes #1749 when moving the `-mtriple` option from the dub config
file to the dub command line.