Support for HWbreak/watchpoint across fork/vfork on arm-native

Support for HWbreak/watchpoint across fork/vfork on arm-native

This patch updates arm native support for hwbreak-/watchpoints to enable
support for hwbreak-/watchpoints across fork/vfork. This involves changes to
hwbreak-/watchpoint insertion mechanism to the modern way, by marking debug
registers as needing update, but only really updating them on resume, which is
necessary for supporting watchpoints in non-stop mode. This also updates a
previously maintained per thread hwbreak-/watchpoint cache to a per process
cache which allows target specific code to come in sync with gdb-linux calls to
threads create/destroy and process fork/exit hooks.