Refactor the NetBSD amd64 gdbserver support

Refactor the NetBSD amd64 gdbserver support

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.

gdbserver/ChangeLog:

	* netbsd-amd64-low.cc (netbsd_x86_64_arch_setup): Remove.
	(netbsd_target_regsets): Now const.
	(the_low_target): Remove.
	(class netbsd_amd64_target, the_netbsd_amd64_target)
	(the_netbsd_target): Add.
	* netbsd-low.cc (netbsd_process_target::post_create_inferior): Call
	low_arch_setup ().
	(netbsd_process_target::fetch_registers)
	(netbsd_process_target::store_registers, initialize_low): Update.
	(the_netbsd_target): Remove.
	* netbsd-low.h (netbsd_target_regsets, netbsd_target_ops)
	(the_low_target, netbsd_tdesc): Remove.
	(netbsd_process_target::get_regs_info)
	(netbsd_process_target::low_arch_setup): Add.