build: fix atomic linking with LTO on s390x

build: fix atomic linking with LTO on s390x

Prior to this change, Fedora and RHEL 9 on s390x would incorrectly pass
the HAVE_CXX11_ATOMIC check. As a consequence, the rest of Ceph would
fail to link because of missing atomic methods "__atomic_load_16",
"__atomic_store_16", "__atomic_compare_exchange_16".

Mark this method so that it is not optimized out, even with LTO. With
this change, s390x properly fails HAVE_CXX11_ATOMIC and falls back to
HAVE_LIBATOMIC.

Move the "BuildRequires: libatomic" RPM line out from the seastar+fedora
conditional and into the main fedora+rhel conditional so that the
HAVE_LIBATOMIC check will pass.

Fixes: https://tracker.ceph.com/issues/54514
Fixes: https://tracker.ceph.com/issues/56492
Co-authored-by: Kaleb S. Keithley <kkeithle@redhat.com>
Signed-off-by: Ken Dreyer <kdreyer@redhat.com>