cmake: check for 64-bit atomic operations directly

780bac671f45ab2c5ef15c9e7d6dff0031673e28 did not actually successfully
compile on any platforms, leading to -latomic always being added
(including on platforms it does not exist on).

Work on #5865.

cmake/ConfigureChecks.cmake

@@ -191,10 +191,10 @@ FIND_PROGRAM(SED sed)
 # https://github.com/fish-shell/fish-shell/issues/5865
 CHECK_CXX_SOURCE_COMPILES("
 #include <atomic>
-struct big { int foo[64]; };
-std::atomic<big> x;
+#include <cstdint>
+std::atomic<uint64_t> x;
 int main() {
-   return x.load().foo[13];
+   return x;
 }"
 LIBATOMIC_NOT_NEEDED)
 IF (NOT LIBATOMIC_NOT_NEEDED)