[Bug libstdc++/94069] [9/10 Regression] doesn't compile unless PTHREAD_RWLOCK_INITIALIZER is defined

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94069

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This causes a bootstrap failure on darwin8 for gcc-9 and master.