[Bug libstdc++/84323] call_once uses TLS even when once_flag is set

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84323

--- Comment #1 from Antony Polukhin <antoshkka at gmail dot com> ---
Fixing this will also resolve Bug 55394, because there'll be no need in linking
with pthread.