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

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

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

	Remove the 'get_ipa_tdesc_idx' linux target op and let a concrete
	linux target define the op by overriding the declaration in
	process_stratum_target.

	* linux-low.h (struct linux_target_ops): Remove the op.
	(class linux_process_target) <get_ipa_tdesc_idx>: Remove.
	* linux-low.cc (linux_process_target::get_ipa_tdesc_idx): Remove.
	* linux-x86-low.cc (class x86_target) <get_ipa_tdesc_idx>: Declare.
	(x86_get_ipa_tdesc_idx): Turn into...
	(x86_target::get_ipa_tdesc_idx): ...this.
	(the_low_target): Remove the op field.
	* linux-ppc-low.cc (class ppc_target) <get_ipa_tdesc_idx>: Declare.
	(ppc_get_ipa_tdesc_idx): Turn into...
	(ppc_target::get_ipa_tdesc_idx): ...this.
	(the_low_target): Remove the op field.
	* linux-s390-low.cc (class s390_target) <get_ipa_tdesc_idx>: Declare.
	(s390_get_ipa_tdesc_idx): Turn into...
	(s390_target::get_ipa_tdesc_idx): ...this.
	(the_low_target): Remove the op field.