hw-breakpoints: modifying generic debug exception to use thread-specific debug registers

This patch modifies the breakpoint exception handler code to use the new
abstract debug register names.

[ fweisbec@gmail.com: fix conflict against kmemcheck ]

[ Impact: refactor and cleanup x86 debug exception handler ]

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>