[Orc] Support atexit in Orc(JITLink)

[Orc] Support atexit in Orc(JITLink)

There is a bug reported at https://bugs.llvm.org/show_bug.cgi?id=48938

After looking through the glibc, I found the `atexit(f)` is the same as `__cxa_atexit(f, NULL, NULL)`. In orc runtime, we identify different JITDylib by their dso_handle value, so that a NULL dso_handle is invalid. So in this patch, I added a `PlatformJDDSOHandle` to ELFNixRuntimeState, and functions which are registered by atexit will be registered at PlatformJD.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D111413