[PATCH 2/2] libstdc++: Remove symbols for new std::call_once implementation [PR 99341]

[Jonathan Wakely]

This removes the new symbols added for the new futex-based
std::call_once implementation. These symbols were new on trunk, so not
in any released version. However, they are already present in some
beta distro releases (Fedora Linux 34) and in Fedora Linux rawhide. This
change can be locally reverted by distros that need to keep the symbols
present until affected packages have been rebuilt.

libstdc++-v3/ChangeLog:

         PR libstdc++/99341
         * config/abi/post/aarch64-linux-gnu/baseline_symbols.txt: Remove
         std::once_flag symbols.
         * config/abi/post/ia64-linux-gnu/baseline_symbols.txt: Likewise.
         * config/abi/post/m68k-linux-gnu/baseline_symbols.txt: Likewise.
         * config/abi/post/riscv64-linux-gnu/baseline_symbols.txt:
         Likewise.
         * config/abi/pre/gnu.ver: Likewise.
         * src/c++11/mutex.cc [_GLIBCXX_HAVE_LINUX_FUTEX]
         (struct __once_flag_compat): Remove.
         (_ZNSt9once_flag11_M_activateEv): Remove.
         (_ZNSt9once_flag9_M_finishEb): Remove.

Tested x86_64-linux, powerpc64-linux and powerpc64le-linux, but not yet
committed (it's not really appropriate for a last-minute Friday
change!)

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