Add minimal and functional NetBSD/amd64 gdbserver

Add minimal and functional NetBSD/amd64 gdbserver

Implement the following functionality: create_inferior,
post_create_inferior, attach, kill, detach, mourn, join, thread_alive,
resume, wait, fetch_registers, store_registers, read_memory, write_memory,
request_interrupt, supports_read_auxv, read_auxv,
supports_hardware_single_step, sw_breakpoint_from_kind,
supports_z_point_type, insert_point, remove_point,
stopped_by_sw_breakpoint, supports_qxfer_siginfo, qxfer_siginfo,
supports_stopped_by_sw_breakpoint, supports_non_stop,
supports_multi_process, supports_fork_events, supports_vfork_events,
supports_exec_events, supports_disable_randomization,
supports_qxfer_libraries_svr4, qxfer_libraries_svr4,
supports_pid_to_exec_file, pid_to_exec_file, thread_name,
supports_catch_syscall.

The only CPU architecture supported: x86_64.

Implement only support for hardware assisted single step and
software breakpoint.

Implement support only for regular X86 registers, thus no FPU.

gdbserver/ChangeLog:

       * netbsd-low.cc: Add.
       * netbsd-low.h: Likewise.
       * netbsd-amd64-low.cc: Likewise.
       * Makefile.in (SFILES): Register "netbsd-low.cc", "netbsd-low.h",
       "netbsd-amd64-low.cc".
       * configure.srv: Add x86_64-*-netbsd*.