util.h: backout 06badf7d10dc and 2c9645c0a582 for big-endian breakage

getbe32 and putbe32 need to behave differently on big-endian and little-endian
systems. On big-endian ones, they should be roughly equivalent to the identity
function with a cast, but on little-endian ones they should reverse the order
of the bytes. That is achieved by the original definition, but
__builtin_bswap32 and _byteswap_ulong, as the names suggest, swap bytes around
unconditionally.

There was no measurable performance improvement, so there's no point adding
extra complexity with even more ifdefs for endianncess.