Re-apply "[llvm-jitlink] Add Process and Platform JITDylibs, ..." with fixes.

This reapplies 3d0dd1a7, which was reverted in df2485b2 due to bot
failures. This patch addresses the issues seen on the bots by disabling two
Linux atexit tests in the ORC runtime whose behavior could not be maintained
now that the ORC runtime is being loaded into a separate Platform JITDylib.
https://github.com/llvm/llvm-project/issues/74641 has been filed to fix the
issue with atexit.