Update to SDL 2.24.0 and add separate macOS UB2 dylib

Update mingw and macOS SDL libraries to 2.24.0. (MSVC .lib files and
Xcode project haven't been updated.)

There is now a separate SDL dylib for macOS Universal Binary 1 and 2
because SDL 2.24.0 dropped support for macOS 10.6 (which is last to
support x86). Documentation says SDL 2.24.0 also dropped macOS 10.7
and 10.8 too. So UB2 is now 10.9 or later while 10.5 to 10.8 live in
UB1.

Compiled using osxcross with macOS 11.3 SDK. Ran "strip -x" on Windows
and macOS libraries. Ad-hoc signed dylib using PyOxidizer rcodesign to
fix digest checksums after strip.

code/SDL2/include/SDL_config_iphoneos.h

@@ -1,6 +1,6 @@
 /*
   Simple DirectMedia Layer
-  Copyright (C) 1997-2021 Sam Lantinga <slouken@libsdl.org>
+  Copyright (C) 1997-2022 Sam Lantinga <slouken@libsdl.org>
 
   This software is provided 'as-is', without any express or implied
   warranty.  In no event will the authors be held liable for any damages
@@ -48,6 +48,7 @@
 /* #undef HAVE_LIBUNWIND_H */
 
 /* C library functions */
+#define HAVE_DLOPEN 1
 #define HAVE_MALLOC 1
 #define HAVE_CALLOC 1
 #define HAVE_REALLOC    1
@@ -59,6 +60,7 @@
 #define HAVE_SETENV 1
 #define HAVE_UNSETENV   1
 #define HAVE_QSORT  1
+#define HAVE_BSEARCH 1
 #define HAVE_ABS    1
 #define HAVE_BCOPY  1
 #define HAVE_MEMSET 1
@@ -116,7 +118,7 @@
 #define HAVE_LROUNDF 1
 #define HAVE_POW    1
 #define HAVE_POWF   1
-#define HAVE_ROUND 1
+#define HAVE_ROUND  1
 #define HAVE_ROUNDF 1
 #define HAVE_SCALBN 1
 #define HAVE_SCALBNF    1
@@ -133,6 +135,7 @@
 #define HAVE_NANOSLEEP  1
 #define HAVE_SYSCONF    1
 #define HAVE_SYSCTLBYNAME 1
+#define HAVE_O_CLOEXEC 1
 
 /* enable iPhone version of Core Audio driver */
 #define SDL_AUDIO_DRIVER_COREAUDIO 1