Actual working multithread support in newlib

[dogtopus]

Some more reversing into threading is needed in order to tell if this is feasible and if yes, how to do it.

• Lock support (atexit() and file related stuff seem to rely on these)
  • Critical sections do not have timeouts so try_acquire is gonna be difficult to do
  • Emulate using semaphores?
• TLS reent (seems to work well but need more testing)
• fdmap hardening (currently it only protects pointer table access not the actual descriptor struct. Go over the code to make sure)