Debugger: Add specific customization hook for dumper initialization

We need to distiguish between start of GDB itself (a.k.a .gdbinit-style
customization) and after the dumper machinery is initialized, which is
nowadays often delayed until the first stop hook.

Change-Id: I40f1e7225c2043b8bcb7d50eef948bb3c9162bb6
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>