Use reinsert_breakpoint for vCont;s

Use reinsert_breakpoint for vCont;s

This patch is to teach GDBserver using software single step to handle
vCont;s.  Simply speaking, if the thread's resume request is resume_step,
install reinsert breakpoint at the next pcs when GDBserver is about to
resume threads.  These reinsert breakpoints of a thread are removed,
when GDBserver gets an event from that thread and reports it back to
GDB.

gdb/gdbserver:

2016-07-21  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (resume_stopped_resumed_lwps): If resume request
	is resume_step, call maybe_hw_step.
	(linux_wait_1): Stop all threads, remove reinsert breakpoints,
	and unstop them.
	(linux_resume_one_lwp_throw): Don't assert the thread has reinsert
	breakpoints or not.
	(proceed_one_lwp): If resume request is resume_step, install
	reinsert breakpoints and call maybe_hw_step.