Add support for NetBSD threads in x86-bsd-nat.c

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

gdb/ChangeLog:

        * x86-bsd-nat.c (x86bsd_dr_get): New variable lwp and pass
        it to the ptrace call.
        * x86-bsd-nat.c (x86bsd_dr_set): Likewise.