Prevent false warnings in memcpy (GCC 12)

GCC 12 tree-loop-distribute-patterns generates false warnings of
-Warray-bounds, -Wstringop-overflow, or -Wstringop-overread in memcpy()
and memcpy_cpu() in static/inline cases for code that obviously prevents
its execution by invariant checking. On -O3, even more warnings are
produced.

repos/base/include/cpu/string.h

@@ -25,7 +25,10 @@ namespace Genode {
 	 * \param size  number of bytes to copy
 	 *
 	 * \return      number of bytes not copied
+	 *
+	 * The compiler attribute prevents array-bounds warnings with gcc 12.3.
 	 */
+	__attribute((optimize("no-tree-loop-distribute-patterns")))
 	inline size_t memcpy_cpu(void * dst, const void * src, size_t size)
 	{
 		using word_t = unsigned long;

repos/base/include/util/string.h

@@ -190,6 +190,9 @@ namespace Genode {
 	 */
 	inline void *memcpy(void *dst, const void *src, size_t size)
 	{
+		if (!size)
+			return dst;
+
 		char *d = (char *)dst, *s = (char *)src;
 		size_t i;