x86/coredump: Use pr_reg size, rather that TIF_IA32 flag

Killed PR_REG_SIZE and PR_REG_PTR macro as we can get regset size
from regset view.
I wish I could also kill PRSTATUS_SIZE nicely.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Cc: 0x7f454c46@gmail.com
Cc: linux-mm@kvack.org
Cc: luto@kernel.org
Cc: gorcunov@openvz.org
Cc: xemul@virtuozzo.com
Link: http://lkml.kernel.org/r/20160905133308.28234-5-dsafonov@virtuozzo.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>