hw-breakpoints: use the new wrapper routines to access debug registers in process/thread code

This patch enables the use of abstract debug registers in
process-handling routines, according to the new hardware breakpoint
Api.

[ Impact: adapt thread breakpoints handling code to the new breakpoint Api ]

Original-patch-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>