Merge #20938: build: fix linking against -latomic when building for riscv

54ce4fa build: improve macro for testing -latomic requirement (fanquake)
2c010b9 add std::atomic include to bitcoin-util.cpp (fanquake)

Pull request description:

  Since the merge of #19937, riscv builds have been failing, due to a link issue with [`std::atomic_exchange`](https://en.cppreference.com/w/cpp/atomic/atomic_exchange) in `bitcoin-util`:
  ```bash
    CXXLD    bitcoin-util
  bitcoin_util-bitcoin-util.o: In function `grind_task':
  /home/ubuntu/build/bitcoin/distsrc-riscv64-linux-gnu/src/bitcoin-util.cpp:98: undefined reference to `__atomic_exchange_1'
  collect2: error: ld returned 1 exit status

  ```

  We have a [macro](https://github.com/bitcoin/bitcoin/blob/master/build-aux/m4/l_atomic.m4) that tries to determine when `-latomic` is required, however it doesn't quite work well enough, as it's currently determining it isn't needed:
  ```bash
  ./autogen.sh
  ./configure --prefix=/home/ubuntu/bitcoin/depends/riscv64-linux-gnu
  ...
  checking whether std::atomic can be used without link library... yes
  ```

  This PR adds a call to `std::atomic_exchange` to the macro, which will get us properly linked against `-latomic` on riscv:
  ```bash
  checking whether std::atomic can be used without link library... no
  checking whether std::atomic needs -latomic... yes
  ```

  Also adds an `<atomic>` include to `bitcoin-util.cpp`.

ACKs for top commit:
  laanwj:
    Tested ACK 54ce4fa

Tree-SHA512: 963c875097ee96b131163ae8109bcf8fecf4451d20faa2f3d223f9938ea3d8d1ed5604e12ad82c2b4b1c605fd293a9b6b08fefc00dd3e68d09c49e95029c6f50

Author: laanwj

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;

src/bitcoin-util.cpp

@@ -25,6 +25,7 @@
 #include <util/system.h>
 #include <util/translation.h>
 
+#include <atomic>
 #include <functional>
 #include <memory>
 #include <stdio.h>