sys/bswap.h in order to pick up the MD inline routines and the constant
folding definitions in the right order.
Code can include either sys/bswap.h or machine/bswap.h with the same effect.
the processor family now. x86_64 is kept as the MACHINE_ARCH value,
since it's already widely used (by e.g. the toolchain, etc), and
by other operating systems.