[Bug libstdc++/67408] assumes that __gthread_mutex_t and__gthread_recursive_mutex_t are the same types

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67408

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There's no need to test on linux, I can do that myself. If it works on your
non-pthreads target I'll commit your patch.