Add support for C++ std::thread and derivatives

[Mefiresu]

Implements dkp's gthread interface using LWP functions.
With this, std::{thread,mutex,recursive_mutex,condition_variable} are now supported.

Note that some parts of the gthread interface are not yet implemented (like gthread_key_*), but it doesn't seem to impact functionality.

[Mefiresu]

Force pushed to add __gthread_once implementation required for some static constructor initializations (heavily based on WUT).
Verified to be working correctly by porting fheroes2 + some manual tests.

[fincs]

libogc v3.0.0 & devkitPPC r49 released, with pthread/C11 thread/C++ std::thread support.