Fix issues with RGB565 color conversion on big endian machines. The RGB565 routines...

Fix issues with RGB565 color conversion on big endian machines.  The RGB565 routines are now abstracted in a separate file, with separate little-endian and big-endian versions defined at compile time through the use of macros (this is similar to how the colorspace extension routines work.)  This allows big-endian machines to take advantage of the same performance optimizations as little-endian machines, and it retains the performance on little-endian machines, since the conditional branch for endianness is at a very coarse-grained level.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/branches/1.4.x@1399 632fc199-4ca6-4c93-a231-07263d6284db