[Bug libstdc++/86846] [9 Regression] ld: (Warning) Unsatisfied symbol "__atomic_exchange_8" in libstdc++.sl

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86846

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. this doesn't need compare and exchange, just exchange.

I'll fix this early next week.