[Bug libstdc++/64203] shared_mutex compile errors on bare-metal targets

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64203

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 34208
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34208&action=edit
fix config macros for shared_lock

Does this fix it?