gdb: add inferior_pre_detach observable

gdb: add inferior_pre_detach observable

Add an observable notified in target_detach just before calling the
detach method on the inferior's target stack.  This allows observer to
do some work on the inferior while it's still ptrace-attached, in the
case of a native Linux inferior.  Specifically, the amd-dbgapi target
will need it in order to call amd_dbgapi_process_detach before the
process gets ptrace-detached.

Change-Id: I28b6065e251012a4c2db8a600fe13ba31671e3c9
Approved-By: Andrew Burgess <aburgess@redhat.com>