Common (Base64): corrects byte order on big-endian hosts (#2470)

ffBase64EncodeRaw unconditionally byte-swapped the input word via
__builtin_bswap32, which only produces the intended big-endian byte
layout on little-endian hosts. On big-endian hosts the word is already
in the correct order, so the swap corrupted the encoded output.

Guard the swap with __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ so the
encoder is correct on both endiannesses.

src/common/impl/base64.c

@@ -6,7 +6,13 @@ void ffBase64EncodeRaw(uint32_t size, const char* str, uint32_t* out_size, char*
     char* out = output;
     const char* ends = str + (size - size % 3);
     while (str != ends) {
-        uint32_t n = __builtin_bswap32(*(uint32_t*) str);
+        uint32_t n = *(uint32_t*) str;
+        #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+        // The 3 input bytes must be laid out big-endian (str[0] in the most
+        // significant position). On little-endian hosts swap; on big-endian
+        // hosts the word is already in the right order.
+        n = __builtin_bswap32(n);
+        #endif
         *out++ = chars[(n >> 26) & 63];
         *out++ = chars[(n >> 20) & 63];
         *out++ = chars[(n >> 14) & 63];