kgdb: x86: Return all segment registers also in 64-bit mode

Even if the content is always 0, gdb expects us to return also ds,
es, fs, and gs while in x86-64 mode. Do this to avoid ugly errors on
"info registers".

[jason.wessel@windriver.com: adjust NUMREGBYTES for two new regs]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>