Allow optimizing memcpy and memset where possible

Modern C runtimes have well optimized memset and memcpy, so use those instead of dispatching into SDL's versions. In addition, some compilers can analyze memset and memcpy calls and directly turn them into optimized assembly.

src/stdlib/SDL_memmove.c

@@ -0,0 +1,55 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2024 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
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "SDL_internal.h"
+
+
+#ifdef SDL_memmove
+#undef SDL_memmove
+#endif
+#if SDL_DYNAMIC_API
+#define SDL_memmove SDL_memmove_REAL
+#endif
+void *SDL_memmove(SDL_OUT_BYTECAP(len) void *dst, SDL_IN_BYTECAP(len) const void *src, size_t len)
+{
+#ifdef __GNUC__
+    /* Presumably this is well tuned for speed. */
+    return __builtin_memmove(dst, src, len);
+#elif defined(HAVE_MEMMOVE)
+    return memmove(dst, src, len);
+#else
+    char *srcp = (char *)src;
+    char *dstp = (char *)dst;
+
+    if (src < dst) {
+        srcp += len - 1;
+        dstp += len - 1;
+        while (len--) {
+            *dstp-- = *srcp--;
+        }
+    } else {
+        while (len--) {
+            *dstp++ = *srcp++;
+        }
+    }
+    return dst;
+#endif /* HAVE_MEMMOVE */
+}
+