Slow attach times due to protecting stack/heap allocations

Created by: Carrotman42

NOTE: the following describes an incorrect solution to the problem of slow attach times. See further down for details about this issue.

During attach on Unix, the thread executing os_take_over_all_unknown_threads:

1. obtains the thread_initexit_lock
2. (does some other stuff)
3. calls thread_signal to all other threads
4. unlocks thread_initexit_lock
5. waits on each threads' "event" one at a time until all threads are suspended

All threads obtain thread_initexit_lock during their attach logic, and I believe signaling all threads and then unlocking the lock causes contention on thread_initexit_lock. With 3000 threads in my simple repro, attaching takes 70 seconds whereas the following implementation finishes in under a second:

1. obtains the thread_initexit_lock
2. (does some other stuff)
3. unlocks thread_initexit_lock
4. for each thread: 4.1) signal that thread 4.2) wait for that thread's "event"