AK: Add feature detection for -fblocks and -fobjc-arc

Author: https://github.com/ADKaster
Commit: https://github.com/LadybirdBrowser/ladybird/commit/0c2f434e690
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3963
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/bugaevc

AK/Platform.h

@@ -265,6 +265,14 @@
 #    define LSAN_UNREGISTER_ROOT_REGION(base, size)
 #endif
 
+#if __has_feature(blocks)
+#    define AK_HAS_BLOCKS
+#endif
+
+#if __has_feature(objc_arc)
+#    define AK_HAS_OBJC_ARC
+#endif
+
 #ifndef AK_OS_SERENITY
 #    ifdef AK_OS_WINDOWS
 // FIXME: No idea where to get this, but it's 4096 anyway :^)