[libc++abi] Overhaul test_exception_storage.pass.cpp

I'm making a change in this area (https://reviews.llvm.org/D138461), so update the test:
* Add proper synchronization instead of a sleep.
* Avoid some unnecessary size_t casts.
* Spawn the number of hardware threads instead of 10.
* Check that `__cxa_get_globals` and `__cxa_get_globals_fast` return
  the same values.
* Split the test in with-threads and without-threads tests to simplify
  the code.

Differential Revision: https://reviews.llvm.org/D138460



Co-authored-by: Louis Dionne <ldionne.2@gmail.com>