__ghtread_recursive_mutex_destroy missing

[Jonathan Wakely]

The gthreads portability layer is missing a function for destroying a
__ghtread_recursive_mutex object.

For pthreads-based models the recursive mutex type is the same as the
normal mutex type so __gthread_mutex_destroy handles both, but they're
distinct types for (at least) gthr-win32.h, so we can't properly
cleanup recursive mutexes in libstdc++

Any objections if I prepare a patch to add
__gthread_recursive_mutex_destroy to each gthr header?