AltiVec SIMD implementation of fast forward DCT


git-svn-id: svn://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1405 632fc199-4ca6-4c93-a231-07263d6284db
The AltiVec code actually works on 32-bit PowerPC platforms as well, so change the "powerpc64" token to "powerpc".  Also clean up the shift code, which wasn't building properly on OS X.


git-svn-id: svn://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1406 632fc199-4ca6-4c93-a231-07263d6284db