Introduce process_stratum_target

Introduce process_stratum_target

This adds a base class that all process_stratum targets inherit from.

default_thread_address_space/default_thread_architecture only make
sense for process_stratum targets, so they are transformed to
process_stratum_target methods/overrides.

gdb/ChangeLog:
2018-11-30  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMMON_SFILES): Add process-stratum-target.c.
	* bsd-kvm.c: Include "process-stratum-target.h".
	(bsd_kvm_target): Now inherits from process_stratum_target.
	(bsd_kvm_target::bsd_kvm_target): Default it.
	* corelow.c: Include "process-stratum-target.h".
	(core_target): Now inherits from process_stratum_target.
	(core_target::core_target): Don't set to_stratum here.
	* inf-child.c (inf_child_target::inf_child_target): Delete.
	* inf-child.h: Include "process-stratum-target.h".
	(inf_child_target): Inherit from process_stratum_target.
	(inf_child_target) <inf_child_target>: Default it.
	<can_async_p, supports_non_stop, supports_disable_randomization>:
	Delete overrides.
	* process-stratum-target.c: New file.
	* process-stratum-target.h: New file.
	* remote-sim.c: Include "process-stratum-target.h".
	(gdbsim_target): Inherit from process_stratum_target.
	<gdbsim_target>: Default it.
	* remote.c: Include "process-stratum-target.h".
	(remote_target): Inherit from process_stratum_target.
	<remote_target>: Default it.
	* target.c (default_thread_address_space)
	(default_thread_architecture): Delete.
	* target.h (target_ops) <thread_architecture>: Now returns NULL by
	default.
	<thread_address_space>: Ditto.
	* test-target.h: Include "process-stratum-target.h" instead of
	"target.h".
	(test_target_ops): Inherit from process_stratum_target.
	<test_target_ops>: Default it.
	* tracefile.c (tracefile_target::tracefile_target): Delete.
	* tracefile.h: Include "process-stratum-target.h".
	(tracefile_target): Inherit from process_stratum_target.
	<tracefile_target>: Default it.
	* target-delegates.c: Regenerate.