Add support for NetBSD threads in arm-nbsd-nat.c

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

gdb/ChangeLog:

	* arm-nbsd-nat.c (fetch_register): New variable lwp and pass
	it to the ptrace call.
	* arm-nbsd-nat.c (fetch_fp_register): Likewise.
	* arm-nbsd-nat.c (fetch_fp_regs): Likewise.
	* arm-nbsd-nat.c (store_register): Likewise.
	* arm-nbsd-nat.c (store_regs): Likewise.
	* arm-nbsd-nat.c (store_fp_register): Likewise.
	* arm-nbsd-nat.c (store_fp_regs): Likewise.