misc/wasm: add stub for fs.openSync for browsers

This stub is necessary so the time package can fail to load
the timezone files in a nice way. It transitively makes the
log package work in browsers.

Change-Id: I4d360df82989d9b40cd31bb4508a6d057534443e
Reviewed-on: https://go-review.googlesource.com/118977Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/wasm/wasm_exec.js

@@ -41,6 +41,11 @@
 				}
 				return buf.length;
 			},
+			openSync(path, flags, mode) {
+				const err = new Error("not implemented");
+				err.code = "ENOSYS";
+				throw err;
+			},
 		};
 	}