Auto merge of #4424 - Xanewok:process-builder-api, r=alexcrichton

Add `fn args_replace` and `fn get_program` to `ProcessBuilder`

`Executor::exec()` provides a `ProcessBuilder` argument representing compiler call and if someone wants to use it or modify it for their own purposes, they have to (partially) recreate it themselves.

This:
* makes the argument modification possible (via `args_replace`, analogous to `env_remove`)
* exposes the program that will be run (via `get_program`, in case `rustc` is overriden via wrapper etc.).

r? @alexcrichton