This commit removes the `Wasmtime*` family of ABIs from Cranelift. These
were originally added to support multi-value in Wasmtime via the
`TypedFunc` API, but they should now no longer be necessary. In general
this is a higher-level Wasmtime concern than something all backends of
Cranelift should have to deal with.

Today with recent refactorings it's possible to remove the reliance on
ABI details for multi-value and instead codify it directly into the
Cranelift IR generated. For example wasm calls are able to have a
"purely internal" ABI which Wasmtime's Rust code doesn't see at all, and
the Rust code only interacts with the native ABI. The native ABI is
redefined to be what the previous Wasmtime ABIs were, which is to return
the first of a 2+ value return through a register (native return value)
and everything else through a return pointer.