[Ada/ravenscar] New name for active-thread symbol

[Ada/ravenscar] New name for active-thread symbol

The GNAT Ravenscar implementation has recently been enhanced to allow
programs using that runtime to run on multi-cpu systems.  One of the
changes that were made is that the name of the symbol we use to
determine which task is the currently-running task has changed.

This patch enhances the debugger to use the new name, and fallback on
the old name if not found (this is for compatibility with older versions
of the compiler/runtime).

gdb/ChangeLog:

        * ravenscar-thread.c (running_thread_name): Change value.
        (read_thread_id): Remove advance declaration.
        (get_running_thread_msymbol): New function.
        (has_ravenscar_runtime): Use get_running_thread_msymbol to
        compute msym_running_thread.
        (get_running_thread_id): Renames read_thread_id. Slight modifications
        to not take any argument anymore, using get_running_thread_msymbol
        to determine which symbol to use instead.
        (ravenscar_running_thread): Use get_running_thread_id instead of
        read_thread_id.