Add support for NetBSD threads in m68k-bsd-nat.c

NetBSD ptrace(2) accepts thread id (LWP) as the 4th argument for threads.

gdb/ChangeLog:

        * m68k-bsd-nat.c (fetch_registers): New variable lwp and pass
        it to the ptrace call.
        * m68k-bsd-nat.c (store_registers): Likewise.