[ORC-RT] Refactor ORC runtime CMake for future test tool(s).

We want to move functionality from the LLVM ORCTargetProcess library into the
ORC runtime, and this will mean implementing remote-executor testing tools
(like llvm-jitlink-executor and lli-child-target) in the ORC runtime.

This patch refactors the ORC runtime build system to introduce an
add_orc_tool function that can be used to add new test tools. The code is
modeled on existing functions for adding unit tests.

A placeholder orc-rt-executor tool and test are added to verify that the
config changes behave as expected.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D133084