message: build: improve macro for testing -latomic requirement
  riscv builds are currently failing because -latomic isn't being linked
  against, when it is needed:
  ```bash
  /home/ubuntu/build/bitcoin/distsrc-riscv64-linux-gnu/src/bitcoin-util.cpp:98: undefined reference to `__atomic_exchange_1'
  ```
  This exteneds our macro to ensure that -latomic is linked against when
  required.
diff: @@ -14,6 +14,9 @@ m4_define([_CHECK_ATOMIC_testbody], [[
  #include <cstdint>
  int main() {
  + std::atomic<bool> lock{true};
  + std::atomic_exchange(&lock, false);
  +
  std::atomic<int64_t> a{};
  int64_t v = 5;
  