syscall/js: make wasm_exec.js compatible with Webpack

In Webpack, require("fs") will always be empty. This behavior throws an error: "fs.writeSync is not function". It happens when you did "fmt.Println".
This PR avoids such problem and use polyfill in wasm_exec.js on Webpack.

Change-Id: I55f2c75ce86b7f84d2d92e8e217b5decfbe3c8a1
GitHub-Last-Rev: aecc847e3f9d5617ea4b00196ef2810c2458f085
GitHub-Pull-Request: golang/go#35805
Reviewed-on: https://go-review.googlesource.com/c/go/+/208600


Reviewed-by: Richard Musiol <neelance@gmail.com>