cmake: fix risc-v build failure

riscv64 is failing due to missing atomic ops in gcc 12.
This commit works around the problem by linking against libatomic.

It can be reverted when we switch to gcc 13 (NixOS#268097)

pkgs/by-name/cm/cmake/package.nix

@@ -138,6 +138,8 @@ stdenv.mkDerivation (finalAttrs: {
     "CFLAGS=-D_FILE_OFFSET_BITS=64"
     "CXXFLAGS=-D_FILE_OFFSET_BITS=64"
   ]
+  # Workaround missing atomic ops with gcc <13
+  ++ lib.optional stdenv.hostPlatform.isRiscV "LDFLAGS=-latomic";
   ++ [
     "--"
     # We should set the proper `CMAKE_SYSTEM_NAME`.