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

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

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

	Turn the 'prepare_to_resume' linux target op into a method of
	linux_process_target.

	* linux-low.h (struct linux_target_ops): Remove the op.
	(class linux_process_target) <low_prepare_to_resume>: Declare.
	* linux-low.cc (linux_process_target::low_prepare_to_resume):
	Define.

	Update the callers below:

	(linux_process_target::resume_one_lwp_throw)
	(linux_process_target::low_prepare_to_resume)

	* linux-x86-low.cc (class x86_target) <low_prepare_to_resume>:
	Declare.
	(x86_target::low_prepare_to_resume): Define.
	(the_low_target): Remove the op field.
	* linux-aarch64-low.cc (class aarch64_target)
	<low_prepare_to_resume>: Declare.
	(aarch64_target::low_prepare_to_resume): Define.
	(the_low_target): Remove the op field.
	* linux-arm-low.cc (class arm_target) <low_prepare_to_resume>:
	Declare.
	(arm_prepare_to_resume): Turn into...
	(arm_target::low_prepare_to_resume): ...this.
	(the_low_target): Remove the op field.
	* linux-mips-low.cc (class mips_target) <low_prepare_to_resume>:
	Declare.
	(mips_linux_prepare_to_resume): Turn into...
	(mips_target::low_prepare_to_resume): ...this.
	(the_low_target): Remove the op field.
	* linux-bfin-low.cc (the_low_target): Remove the op field.
	* linux-crisv32-low.cc (the_low_target): Ditto.
	* linux-m32r-low.cc (the_low_target): Ditto.
	* linux-m68k-low.cc (the_low_target): Ditto.
	* linux-ppc-low.cc (the_low_target): Ditto.
	* linux-s390-low.cc (the_low_target): Ditto.
	* linux-sh-low.cc (the_low_target): Ditto.
	* linux-tic6x-low.cc (the_low_target): Ditto.
	* linux-tile-low.cc (the_low_target): Ditto.
	* linux-xtensa-low.cc (the_low_target): Ditto.