misc/wasm: free up memory on exit

Private fields of the Go class are not used any more after the program
has exited. Delete them to allow JavaScript's garbage collection to
clean up the WebAssembly instance.

Updates #26193.

Change-Id: I349784a49eaad0c22ceedd4f859df97132775537
Reviewed-on: https://go-review.googlesource.com/122296
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/wasm/wasm_exec.js

@@ -177,8 +177,12 @@
 				go: {
 					// func wasmExit(code int32)
 					"runtime.wasmExit": (sp) => {
+						const code = mem().getInt32(sp + 8, true);
 						this.exited = true;
-						this.exit(mem().getInt32(sp + 8, true));
+						delete this._inst;
+						delete this._values;
+						delete this._refs;
+						this.exit(code);
 					},
 
 					// func wasmWrite(fd uintptr, p unsafe.Pointer, n int32)