gdbserver/linux-low: turn 'supports_hardware_single_step' into a method

gdbserver/linux-low: turn 'supports_hardware_single_step' into a method

All the linux low targets except arm define the
'supports_hardware_single_step' op to return true.  Hence, we override
the method to return true in linux_process_target, and remove the
definitions in all the linux low targets but arm.

gdbserver/ChangeLog:
2020-04-02  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	Remove the 'supports_hardware_single_step' linux target op and
	override the process_stratum_target's op definition in
	linux_process_target to return true.

	* linux-low.h (struct linux_target_ops): Remove the op.
	(class linux_process_target) <finish_step_over>
	<maybe_hw_step>: Declare.
	* linux-low.cc (can_hardware_single_step): Remove.
	(maybe_hw_step): Turn into...
	(linux_process_target::maybe_hw_step): ...this.
	(finish_step_over): Turn into...
	(linux_process_target::finish_step_over): ...this.
	(linux_process_target::supports_hardware_single_step): Update
	to return true.

	Update the callers below.

	(linux_process_target::single_step)
	(linux_process_target::resume_one_lwp_throw)

	* linux-arm-low.cc (class arm_target)
	<supports_hardware_single_step>: Declare.
	(arm_supports_hardware_single_step): Turn into...
	(arm_target::supports_hardware_single_step): ...this.
	(the_low_target): Remove the op field.
	* linux-x86-low.cc (x86_supports_hardware_single_step): Remove.
	(the_low_target): Remove the op field.
	* linux-aarch64-low.cc (aarch64_supports_hardware_single_step):
	Remove.
	(the_low_target): Remove the op field.
	* linux-bfin-low.cc (bfin_supports_hardware_single_step): Remove.
	(the_low_target): Remove the op field.
	* linux-crisv32-low.cc (cris_supports_hardware_single_step): Remove.
	(the_low_target): Remove the op field.
	* linux-m32r-low.cc (m32r_supports_hardware_single_step): Remove.
	(the_low_target): Remove the op field.
	* linux-m68k-low.cc (m68k_supports_hardware_single_step): Remove.
	(the_low_target): Remove the op field.
	* linux-ppc-low.cc (ppc_supports_hardware_single_step): Remove.
	(the_low_target): Remove the op field.
	* linux-s390-low.cc (s390_supports_hardware_single_step): Remove.
	(the_low_target): Remove the op field.
	* linux-sh-low.cc (sh_supports_hardware_single_step): Remove.
	(the_low_target): Remove the op field.
	* linux-tic6x-low.cc (tic6x_supports_hardware_single_step): Remove.
	(the_low_target): Remove the op field.
	* linux-tile-low.cc (tile_supports_hardware_single_step): Remove.
	(the_low_target): Remove the op field.
	* linux-xtensa-low.cc (xtensa_supports_hardware_single_step):
	Remove.
	(the_low_target): Remove the op field.