kgdb,x86: Individual register get/set for x86

Implement the ability to individually get and set registers for kdb
and kgdb for x86.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: H. Peter Anvin <hpa@zytor.com>
CC: Ingo Molnar <mingo@redhat.com>
CC: x86@kernel.org