[3PP/MacOS] Fix ffmpeg and sha256 build

ffmpeg: macOS uses "_" symbol prefix (Mach-O), has no <malloc.h>,
and doesn't support .data.rel.ro sections. Add __APPLE__ guards
in ffmpeg-xenia/config.h.

sha256: macOS provides endian macros in <machine/endian.h> instead
of <endian.h>.

third_party/crypto/sha256.cpp

@@ -9,8 +9,12 @@
 
 // big endian architectures need #define __BYTE_ORDER __BIG_ENDIAN
 #ifndef _MSC_VER
+#ifdef __APPLE__
+#include <machine/endian.h>
+#else
 #include <endian.h>
 #endif
+#endif
 
 namespace sha256 {