LibTest: Prevent -dead_strip from removing assertion handler

The ak_assertion_handler symbol is referenced only through a weak
symbol in AK/Assertions.cpp. With -dead_strip on macOS, the linker
strips the strong definition, breaking EXPECT_DEATH tests which rely
on the handler to longjmp back instead of crashing.

Add retain and used attributes to prevent the linker from removing it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Libraries/LibTest/AssertionHandler.cpp

@@ -38,7 +38,7 @@ static void assertion_handler_impl(char const*)
 #if defined(AK_OS_WINDOWS)
 #    define EXPORT __declspec(dllexport)
 #else
-#    define EXPORT __attribute__((visibility("default")))
+#    define EXPORT __attribute__((visibility("default"), retain, used))
 #endif
 
 extern "C" EXPORT void ak_assertion_handler(char const* message);