Implement Target hook to retrieve size of critical section.

Mentioned this being a pain in https://github.com/D-Programming-Language/dmd/pull/2058

This should make it so that you can cross compile between targets safely, as you are not relying on ```sizeof(critsectype)``` to get the size of pthread_mutex_t / CRITICAL_SECTION / other.

I recommend scrutinous review, as the returned values are part based off what already existed, and what I quickly gathered from either the implementation of pthread_mutex_t in a random pthread.h on the internet or in core.sys.posix.