build: Fix check whether `-latomic` needed #29177

Clang >=15 still might need linking against libatomic.

We use std::atomic<std::chrono::seconds>::compare_exchange_strong in net_processing.cpp.

Addresses the #29165 (review).

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Testing on a fresh install of Ubuntu 24.04 Noble: export DEBIAN_FRONTEND=noninteractive && apt update && apt install curl wget htop git vim ccache -y && git clone https://github.com/bitcoin/bitcoin.git --depth=1 && cd bitcoin && git fetch origin --depth=1 f8ca1357c8205ceff732dcfb0d2bad79b40b611b && git checkout f8ca1357c8205ceff732dcfb0d2bad79b40b611b && apt install build-essential libtool autotools-dev automake pkg-config bsdmainutils python3 make automake cmake curl clang-17 llvm-17 g++-multilib libtool binutils-gold bsdmainutils pkg-config python3 patch bison -y && ( cd depends && make DEBUG=1 HOST=i686-pc-linux-gnu NO_QT=1 NO_WALLET=1 NO_ZMQ=1 NO_UPNP=1 NO_NATPMP=1 -j $(nproc) ) && ./autogen.sh && CONFIG_SITE="$PWD/depends/i686-pc-linux-gnu/share/config.site" ./configure CC='clang-17 -m32' CXX='clang++-17 -m32' --enable-fuzz --with-sanitizers=fuzzer && make -j $(nproc)

On master:

  CXXLD    test/fuzz/fuzz
/usr/bin/ld: libbitcoin_node.a(libbitcoin_node_a-net_processing.o): in function `(anonymous namespace)::PeerManagerImpl::BlockConnected(ChainstateRole, std::shared_ptr<CBlock const> const&, CBlockIndex const*)':
net_processing.cpp:(.text._ZN12_GLOBAL__N_115PeerManagerImpl14BlockConnectedE14ChainstateRoleRKSt10shared_ptrIK6CBlockEPK11CBlockIndex[_ZN12_GLOBAL__N_115PeerManagerImpl14BlockConnectedE14ChainstateRoleRKSt10shared_ptrIK6CBlockEPK11CBlockIndex]+0x1ef): undefined reference to `__atomic_compare_exchange'
/usr/bin/ld: libbitcoin_node.a(libbitcoin_node_a-net_processing.o): in function `(anonymous namespace)::PeerManagerImpl::SendMessages(CNode*)':
net_processing.cpp:(.text._ZN12_GLOBAL__N_115PeerManagerImpl12SendMessagesEP5CNode[_ZN12_GLOBAL__N_115PeerManagerImpl12SendMessagesEP5CNode]+0x6755): undefined reference to `__atomic_compare_exchange'
clang++-17: error: linker command failed with exit code 1 (use -v to see invocation)

This pull:

(all good)

lgtm ACK f8ca1357c8205ceff732dcfb0d2bad79b40b611b

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Guix builds (on x86_64)

ACK f8ca1357c8205ceff732dcfb0d2bad79b40b611b