libretro, fix apple os builds

platform/libretro/libretro.c

@@ -211,6 +211,16 @@ void cache_flush_d_inval_i(void *start, void *end)
 #endif
 }
 
+#ifdef __MACH__
+/* calls to this may be generated by the compiler, but it's missing in libc? */
+void __clear_cache(void *start, void *end)
+{
+   size_t len = (char *)end - (char *)start;
+   sys_dcache_flush(start, len);
+   sys_icache_invalidate(start, len);
+}
+#endif
+
 #ifdef _WIN32
 /* mmap() replacement for Windows
  *