Handle CPU offset for Ravenscar

Handle CPU offset for Ravenscar

The Ravenscar support assumes that the thread ID is the same as the
CPU ID that appears in the Ada task structure.  However, on some
systems, gdbserver will report thread IDs that are off by some
constant.  This can happen, e.g., with qemu in a scenario where there
is an additional (unreported) CPU in the emulation.

The Ada Ravenscar runtimes have been modified to store this offset in
a global variable.  This patch changes gdb to read this variable, when
it exists, and apply the offset to the base CPU ID.  This fixes some
crashes that otherwise occur.

2020-12-11  Tom Tromey  <tromey@adacore.com>

	* ada-tasks.c (struct ada_tasks_pspace_data) <cpu_id_offset>: New
	field.
	(ada_get_tcb_types_info): Look for __gnat_gdb_cpu_first_id.
	(read_atcb): Use cpu_id_offset.