Switch current_thread to lwp's thread in install_software_single_step_breakpoints

Switch current_thread to lwp's thread in install_software_single_step_breakpoints

install_software_single_step_breakpoints has parameter lwp, but still
need to switch to current_thread.  In order to simplify its caller,
we do the current_thread save/restore inside install_software_single_step_breakpoints.

gdb/gdbserver:

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

	* gdbthread.h (make_cleanup_restore_current_thread): Declare.
	* inferiors.c (do_restore_current_thread_cleanup): New function.
	(make_cleanup_restore_current_thread): Likewise.
	* linux-low.c (install_software_single_step_breakpoints): Call
	make_cleanup_restore_current_thread.  Switch current_thread to
	thread.