[MIPS] Use lwpid from lwp_info instead of inferior_ptid

[MIPS] Use lwpid from lwp_info instead of inferior_ptid

RAJESH reported that GDB gets "Couldn't write debug register: No such
process." on mips64 when GDB attaches to a multi threaded application.

Looks GDB nows PTRACE_GET_WATCH_REGS for inferior_ptid but
PTRACE_SET_WATCH_REGS for lwp->ptid, they may be different.

gdb:

2017-04-28  Yao Qi  <yao.qi@linaro.org>

	* mips-linux-nat.c (mips_linux_new_thread): Get lwpid from
	lwp_info instead of getting from inferior_ptid.