gdb: Implement native dumpcore function for NetBSD

Define supports_dumpcore and dumpcore for NetBSD, that wraps
the ptrace(2) call with the PT_DUMPCORE operation.

gdb/ChangeLog:

       * nbsd-nat.h (nbsd_nat_target::supports_dumpcore)
       (nbsd_nat_target::dumpcore): New declarations.
       * nbsd-nat.c (nbsd_nat_target::supports_dumpcore)
       (nbsd_nat_target::dumpcore): New functions.