add `wasm32-wasi-preview2` target (#457)

add `wasm32-wasi-preview2` target (#457)

Currently, this is identical to the `wasm32-wasi` in all but name.  See #449 for
the next step, which is to incrementally add Preview 2 features,
e.g. `wasi-sockets`.  Per the discussion in that PR, I've split the
`wasi-sysroot/include` directory into per-target directories.  Eventually, we'll
want to build a separate sysroot for each target, but there's currently
uncertainty about how to configure the default sysroot for e.g. clang, so we're
not tackling that yet.

See also #447 for further details.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>