pc64: dont save/restore DS/ES/FS/GS segment registers on syscall or interrupt, they are ignored in long mode.

we do not support 32 bit processes and DS, ES, FS and GS segment
registers are ignored in long mode, so theres no point in saving
and restoring them.
pc64: cleanup syscallentry()

avoid the stack shuffeling and make syscallentry() and
forkret() use the same stack offsets.
libdraw: add missing borderop() (thanks aiju)