[gdbserver] Rename supports_conditional_breakpoints to supports_hardware_single_step

[gdbserver] Rename supports_conditional_breakpoints to supports_hardware_single_step

In my patch https://sourceware.org/ml/gdb-patches/2015-04/msg01110.html
a new target_ops hook supports_conditional_breakpoints was added to
disable conditional breakpoints if target doesn't have hardware single
step.  This patch is to generalize this hook from
supports_conditional_breakpoints to supports_hardware_single_step,
so that the following patch can use it.

gdb/gdbserver:

2015-09-15  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (linux_supports_conditional_breakpoints): Rename
	it to ...
	(linux_supports_hardware_single_step): ... New function.
	(linux_target_ops): Update.
	* lynx-low.c (lynx_target_ops): Set field
	supports_hardware_single_step to target_can_do_hardware_single_step.
	* nto-low.c (nto_target_ops): Likewise.
	* spu-low.c (spu_target_ops): Likewise.
	* win32-low.c (win32_target_ops): Likewise.
	* target.c (target_can_do_hardware_single_step): New function.
	* target.h (struct target_ops) <supports_conditional_breakpoints>:
	Remove.  <supports_hardware_single_step>: New field.
	(target_supports_conditional_breakpoints): Remove.
	(target_supports_hardware_single_step): New macro.
	(target_can_do_hardware_single_step): Declare.
	* server.c (handle_query): Use target_supports_hardware_single_step
	instead of target_supports_conditional_breakpoints.