(v6.x backport) child_process: fix deoptimizing use of arguments

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

benchmark, child_process

Backport of #11535

I've removed changes in execFile(): one cause was not in code yet, another one is relevant only from v8 5.6.

Commit message also was edited: one Refs was removed, the wording was a bit corrected. All other metadata was left untouched.

I've checked the benchmark with the last v6. We need to run CI for the lib.