wasm: execute an 'unreachable' trap inst on panic

This already happened when directly calling panic() but not with some
runtime panics like accessing a slice out of bounds.

src/runtime/runtime_wasm.go

@@ -49,6 +49,7 @@ func align(ptr uintptr) uintptr {
 	return (ptr + 3) &^ 3
 }
 
+// Abort executes the wasm 'unreachable' instruction.
 func abort() {
-	// TODO
+	trap()
 }