cortex_m: implement hit_watchpoint function

cortex_m: implement hit_watchpoint function

this change aims to provide a better gdb debugging experience,
by making gdb understand what's really happening.

before this change when hitting a watchpoint
 - openocd reports "T05" to gdb
 - gdb displays: Program received signal SIGTRAP, Trace/breakpoint trap.

after the change
 - openocd reports "T05watch:20000000;" to gdb
 - gdb displays:
   Hardware watchpoint 1: *0x20000000

   Old value = 16000000
   New value = 170000000
   ...

Change-Id: Iac3a85eadd86663617889001dd04513a4211ced9
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/6181
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>