Initial reftype support in aarch64, modulo safepoints.

This commit adds the inital support to allow reftypes to flow through
the program when targetting aarch64. It also adds a fix to the
`ModuleTranslationState` needed to send R32/R64 types over from the
SpiderMonkey embedding.

This commit does not include any support for safepoints in aarch64
or the `MachInst` infrastructure; that is in the next commit.

This commit also makes a drive-by improvement to `Bint`, avoiding an
unneeded zero-extension op when the extended value comes directly from a
conditional-set (which produces a full-width 0 or 1).

cranelift/wasm/src/state/module_state.rs

@@ -30,6 +30,7 @@ fn cranelift_to_wasmparser_type(ty: Type) -> WasmResult<wasmparser::Type> {
         types::I64 => wasmparser::Type::I64,
         types::F32 => wasmparser::Type::F32,
         types::F64 => wasmparser::Type::F64,
+        types::R32 | types::R64 => wasmparser::Type::ExternRef,
         _ => {
             return Err(WasmError::Unsupported(format!(
                 "Cannot convert Cranelift type to Wasm signature: {:?}",