The patch limits read_buffer_size and read_rnd_buffer_size by 2 GB on all platforms for the following reasons:
  
- I/O code in mysys, code in mf_iocache.c and in some storage engines do not currently work with sizes > 2 GB for those buffers
- even if the above had been fixed, Windows POSIX read() and write() calls are not 2GB-safe, so setting those buffer to sizes > 2GB would not work correctly on 64-bit Windows.