Fix unportable definition of BSWAP64() macro.

Fix unportable definition of BSWAP64() macro.

We have a portable way of writing uint64 constants, but whoever wrote
this macro didn't know about it.

While at it, fix unsafe under-parenthesization of arguments.  That might
be moot, because there are already good reasons not to use the macro on
anything more complicated than a simple variable, but it's still poor
practice.

Per buildfarm warnings.