Software breakpoint removal fixed.

Linux software breakpoint program counter backup now happens
on breakpoint hit.

Fixed bug in breakpoint disable logic.  Breakpoints are currently
created enabled but were not running through the NativeBreakpoint::Enable
logic os the m_enabled flag was not set correctly, causing it to not
get removed correctly.  Review $Z0/z0 set/remove and possibly the
software breakpoint create-and-enable since those are one step right
now.  The Set/Remove and Enable/Disable could be kept separate.

Lots of logging added around GetPC ().  A key generic register lookup
method was not yet implemented.  Pushed that down to base NativeRegisterContext
class since it is implemented in terms of virtual methods.

There is probably more logging on the register reading than is necessary.
We might want to remove some of that (on the thread logging flag) if
it affects performance.