child_process: support execFile

[ahaoboy]

A function very similar to spawn

https://nodejs.org/api/child_process.html#child_processexecfilefile-args-options-callback

llrt's child_process module does not have a corresponding sync function, it would be nice if it could be supported

[nithinkjoy-tech]

Hi @richarddavison ,
I started working on this. Can you please assign this to me?