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

[sandra at codesourcery dot com]

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

--- Comment #4 from Sandra Loosemore <sandra at codesourcery dot com> ---
(In reply to Jonathan Wakely from comment #3)
> How's this one?

Looks better; this version fixes the compile-time errors.