wasmtime: Implement global.{get,set} for externref globals (#1969)

* wasmtime: Implement `global.{get,set}` for externref globals

We use libcalls to implement these -- unlike `table.{get,set}`, for which we
create inline JIT fast paths -- because no known toolchain actually uses
externref globals.

Part of #929

* wasmtime: Enable `{extern,func}ref` globals in the API

crates/runtime/src/lib.rs

@@ -67,3 +67,14 @@ pub fn ref_type() -> wasmtime_environ::ir::Type {
         unreachable!()
     }
 }
+
+/// The Cranelift IR type used for pointer types for this target architecture.
+pub fn pointer_type() -> wasmtime_environ::ir::Type {
+    if cfg!(target_pointer_width = "32") {
+        wasmtime_environ::ir::types::I32
+    } else if cfg!(target_pointer_width = "64") {
+        wasmtime_environ::ir::types::I64
+    } else {
+        unreachable!()
+    }
+}