libstdc++: Move atomic functions to libsupc++ [PR 96657]

libstdc++: Move atomic functions to libsupc++ [PR 96657]

The changes for PR libstdc++/64735 mean that libsupc++ function might
now depend on the __exchange_and_add and __atomic_add functions defined
in config/cpu/*/atomicity.h which is not compiled into libsupc++. This
causes a link failure for some targets when trying to use libsupc++
without the rest of libstdc++.

This patch simply moves the definitions of those functions into
libsupc++ so that they are available there.

libstdc++-v3/ChangeLog:

	PR libstdc++/96657
	* libsupc++/Makefile.am: Add atomicity.cc here.
	* src/c++98/Makefile.am: Remove it from here.
	* libsupc++/Makefile.in: Regenerate.
	* src/c++98/Makefile.in: Regenerate.
	* testsuite/18_support/exception_ptr/96657.cc: New test.