Add multiple-CPU support in ravenscar-thread.c

Add multiple-CPU support in ravenscar-thread.c

This patch reworks the ravenscar-thread layer to remove the
assumption that the target only has 1 CPU. In particular,
when connected to a QEMU target over the remote protocol,
QEMU reports each CPU as one thread. This patch adapts
the ravenscar-thread layer to this, and adds a large comment
explaining the general design of this unit.

gdb/ChangeLog:

        * ada-lang.h (ada_get_task_info_from_ptid): Add declaration.
        * ada-tasks.c (ada_get_task_info_from_ptid): New function.
        * ravenscar-thread.c: Add into comment.
        (base_magic_null_ptid): Delete.
        (base_ptid): Change documentation.
        (ravenscar_active_task): Renames ravenscar_running_thread.
        All callers updated throughout.
        (is_ravenscar_task, ravenscar_get_thread_base_cpu): New function.
        (ravenscar_task_is_currently_active): Likewise.
        (get_base_thread_from_ravenscar_task): Ditto.
        (ravenscar_update_inferior_ptid): Adjust to handle multiple CPUs.
        (ravenscar_runtime_initialized): Likewise.
        (get_running_thread_id): Add new parameter "cpu".  Adjust
        implementation to handle this new parameter.
        (ravenscar_fetch_registers): Small adjustment to use
        is_ravenscar_task and ravenscar_task_is_currently_active in
        order to decide whether to use the target beneath or this
        module's arch_ops.
        (ravenscar_store_registers, ravenscar_prepare_to_store): Likewise.
        (ravenscar_stopped_by_sw_breakpoint): Use
        get_base_thread_from_ravenscar_task to get the underlying
        thread, rather than using base_ptid.
        (ravenscar_stopped_by_hw_breakpoint, ravenscar_stopped_by_watchpoint)
        (ravenscar_stopped_data_address, ravenscar_core_of_thread):
        Likewise.
        (ravenscar_inferior_created): Do not set base_magic_null_ptid.