Replace the pre-C++ construct of netbsd_target_ops, netbsd_regset_info
and netbsd_tdesc with C++ inheritance approach found in the Linux
gdbserver code. Add netbsd_amd64_target, that inherits from the
netbsd_process_target class and add proper singleton object for
the_netbsd_target, initialized from netbsd_amd64_target.
Call low_arch_setup () on post process creation, which sets machine
specific properties of the traced process.
Remove global singleton the_netbsd_target object from the generic
gdbserver code.
This refactoring introduces no functional change from the end-user
point of view.