570cb24498
1. Make the feature test work by excluding known-deficient processors, so we don't have to maintain a complete list of all the processors that support REV and REV16. 2. Don't abuse 'register' to get an effect similar to GCC's +l constraint, but which was unnecessarily restrictive. 3. Fix __swap64md so _x isn't clobbered, breaking 64-bit swaps. 4. Make <byteswap.h> (which declars bswap_16 and friends) use <endian.h> rather than <sys/endian.h>, so we get the machine-dependent implementations. (git cherry-pick of 3cf53d1a7814e1520df09d24b009c16f4f27db0d)