kgdb, x86_64: fix PS CS SS registers in gdb serial

On x86_64 the gdb serial register structure defines the PS (also known
as eflags), CS and SS registers as 4 bytes entities.

This patch splits the x86_64 regnames enum into a 32 and 64 version to
account for the 32 bit entities in the gdb serial packets.

Also the program counter is properly filled in for the sleeping
threads.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>