Provide the "Base CPU" in output of "info task" (if set by runtime).

Provide the "Base CPU" in output of "info task" (if set by runtime).

At the user level, this patch enhances the debugger to print the ID
of the base CPU a task is running on:

        (gdb) info task 3
        Ada Task: 0x13268
        Name: raven1
        Thread: 0x13280
        LWP: 0
 !!!->  Base CPU: 1
        No parent
        Base Priority: 127
        State: Runnable

This new field is only printed when the base CPU is nonzero or, in
other words, if the base CPU info is being provided by the runtime.
For instance, on native systems, where threads/processes can "jump"
from CPU to CPU, the info is not available, and the output of the
command above then remains unchanged.

At the internal level, the real purpose of this change is to prepare
the way for ravenscar-thread to start handling SMP systems. For that,
we'll need to know which CPU each task is running on...  More info
on that in the commit that actually adds support for it.

gdb/ChangeLog:

        * ada-lang.h (struct ada_task_info) <base_cpu>: New field.
        * ada-lang.c (struct atcb_fieldno) <base_cpu>: New field.
        (get_tcb_types_info): Set fieldnos.base_cpu.
        (read_atcb): Set task_info->base_cpu.
        (info_task): Print "Base CPU" info if set by runtime.