Since former ESP32 toolchain versions used POSIX threads, module `pthread` was required. The built-in `cxa_ctor_guards` had to be replaced since they used the `pthread_once` function for singleton objects initialization where the parameter `once` was of incompatible type with that provided by RIOT's `pthread` module. The current ESP32 toolchain version no longer uses POSIX threads. The dependency on module `pthread` as well as according C++ hacks can be removed.
