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.

build-aux/m4/l_atomic.m4

@@ -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;