[llvm-exegesis] Introduce Subprocess Executor Mode

This patch introduces the subprocess executor mode. Currently, this new
mode doesn't do anything fancy, just executing the same code that the
inprocess executor would do, but within a subprocess. This sets up the
ability to add in many more memory-related features in the future.

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D151021