windres uses unsigned long for fwrite'ing 32-bit little-endian integers (DWORD).
 This breaks cross tools on 64-bit and/or bigendian hosts.

When writing res file, resource header (write_res_header() from resres.c) is two
64-bit integers instead of 32 bit.

There's also FIXME in code about that issue (resres.c):

/* FIXME: This file does not work correctly in a cross configuration.
   It assumes that it can use fread and fwrite to read and write
   integers.  It does no swapping.  */