test: Add the original test case for atomic loads of varying sizes

This covers the original testcase from
https://github.com/mstorsjo/llvm-mingw/issues/538.

test/Makefile

@@ -74,7 +74,7 @@ TESTS_C_LINK_DLL = autoimport-main
 TESTS_C_NO_BUILTIN = crt-test
 TESTS_C_ANSI_STDIO = crt-test
 TESTS_C_AS_CPP = crt-test
-TESTS_CPP = hello-cpp global-terminate tlstest-main longjmp-cleanup
+TESTS_CPP = hello-cpp global-terminate tlstest-main longjmp-cleanup atomic-load-sizes
 TESTS_CPP_EXCEPTIONS = hello-exception exception-locale exception-reduced exception-catch-ptr-ref
 TESTS_CPP_STATIC = hello-exception
 TESTS_CPP_DLL = tlstest-lib throwcatch-lib

test/atomic-load-sizes.cpp

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2026 Martin Storsjo
+ *
+ * This file is part of llvm-mingw.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <array>
+#include <atomic>
+#include <cstdint>
+#include <iostream>
+#include <numeric>
+
+int retval = 0;
+
+template <std::size_t size, std::size_t max_size>
+void f() {
+    using type_t = std::array<std::uint8_t, size>;
+    type_t init;
+    for (std::size_t i = 0; i < size; i++)
+        init[i] = i;
+    std::atomic<type_t> atom(init);
+    bool match = atom.load() == init;
+    std::cout << size << " byte atomic: " << (match ? "ok\n" : "FAILED\n");
+    if (!match)
+        retval = 1;
+    if constexpr (size < max_size) {
+        f<size + 1, max_size>();
+    }
+}
+
+int main() {
+    f<1, 32>();
+    return retval;
+}