Update internal_pthread_create to be more robust
Only to atomic_swap, not atomic_cas (unnecessary)
Move lock after checking queue.

No need to grab the lock every time we check the queue since we only start
pulling things out of it if there is anything to pull out anyway.
Trigger thread paused callback instead of blocked
Add more syscalls for the syscall interposition
Add new vcore api call vcore_request_specific

This call allows you to rerequest a specific vcore from the system.
This is useful when you want to yield the core back so as not to waste cpu
cycles, but you want to make sure you get the same one back when you reequest
it again in the future (for purposes of per-vcore structures).