Refactor queries for hardware and software single stepping support in GDBServer.

Refactor queries for hardware and software single stepping support in GDBServer.

Before this patch there was only one call: can_hardware_single_step. Its
implementation was a check on breakpoint_reinsert_addr if NULL it assumed
that the target could hardware single step.

This patch prepares for the case where this is not true anymore.

In order to improve software single stepping in GDBServer the
breakpoint_reinsert_addr operation of targets that had a very simple
software implementation used only for stepping over thread creation events
will be removed.

This will create a case where a target does not support hardware single
step and has the operation breakpoint_reinsert_addr set to NULL, thus
can_hardware_single_step needs to be implemented another way.

A new target operation supports_hardware_single_step is introduced and is
to return true if the target does support such a feature, support for the
feature is manually hardcoded.

Note that the hardware single step support was enabled as per the current
behavior, I did not check if tile for example really has ptrace singlestep
support but since the current implementation assumed it had, I kept it
that way.

No regressions on Ubuntu 14.04 on ARMv7 and x86.
With gdbserver-{native,extended} / { -marm -mthumb }

Compilation tested on: aarch64,arm,bfind,crisv32,m32r,ppc,s390,tic6x,tile,
xtensa.
Not tested : sh.

gdb/gdbserver/ChangeLog:

	* linux-aarch64-low.c (aarch64_supports_hardware_single_step):
	New function.
	(struct linux_target_ops) <supports_hardware_single_step>: Initialize.
	* linux-arm-low.c (arm_supports_hardware_single_step): New function.
	(struct linux_target_ops) <supports_hardware_single_step>: Initialize.
	* linux-bfin-low.c (bfin_supports_hardware_single_step): New function.
	(struct linux_target_ops) <bfin_supports_hardware_single_step>:
	Initialize.
	* linux-crisv32-low.c (cris_supports_hardware_single_step):
	New function.
	(struct linux_target_ops) <supports_hardware_single_step>: Initialize.
	* linux-low.c (can_hardware_single_step): Use
	supports_hardware_single_step.
	(can_software_single_step): New function.
	(start_step_over): Call can_software_single_step.
	(linux_supports_hardware_single_step): New function.
	(struct target_ops) <supports_software_single_step>: Initialize.
	* linux-low.h (struct linux_target_ops)
	<supports_hardware_single_step>: Initialize.
	* linux-m32r-low.c (m32r_supports_hardware_single_step): New function.
	(struct linux_target_ops) <supports_hardware_single_step>: Initialize.
	* linux-ppc-low.c (ppc_supports_hardware_single_step): New function.
	(struct linux_target_ops) <supports_hardware_single_step> Initialize.
	* linux-s390-low.c (s390_supports_hardware_single_step): New function.
	(struct linux_target_ops) <supports_hardware_single_step>: Initialize.
	* linux-sh-low.c (sh_supports_hardware_single_step): New function.
	(struct linux_target_ops) <supports_hardware_single_step>: Initialize.
	* linux-tic6x-low.c (tic6x_supports_hardware_single_step): New function.
	(struct linux_target_ops) <tic6x_supports_hardware_single_step>:
	Initialize.
	* linux-tile-low.c (tile_supports_hardware_single_step): New function.
	(struct linux_target_ops) <tile_supports_hardware_single_step>:
	Initialize.
	* linux-x86-low.c (x86_supports_hardware_single_step) New function.
	(struct linux_target_ops) <supports_hardware_single_step>: Initialize.
	* linux-xtensa-low.c (xtensa_supports_hardware_single_step):
	New function.
	(struct linux_target_ops) <supports_hardware_single_step>: Initialize.
	* target.h (struct target_ops): <supports_software_single_step>:
	New field.
	(target_supports_software_single_step): New macro.