(Ada) Add ravenscar tasking support on AArch64

(Ada) Add ravenscar tasking support on AArch64

This patch adds support for debugging Ravenscar tasks, similar to what
is done for ppc and sparc.

gdb/ChangeLog:

        * aarch64-ravenscar-thread.h, aarch64-ravenscar-thread.c:
        New files.
        * aarch64-tdep.c: #include "aarch64-ravenscar-thread.h".
        (aarch64_gdbarch_init): Add call to register_aarch64_ravenscar_ops.
        * Makefile.in (ALL_64_TARGET_OBS): Add aarch64-ravenscar-thread.o.
        (HFILES_NO_SRCDIR): Add aarch64-ravenscar-thread.h.
        (ALLDEPFILES): Add aarch64-ravenscar-thread.c.
        * configure.tgt (cpu_obs) [aarch64*-*-*]: Add ravenscar-thread.o
        and aarch64-ravenscar-thread.o.
        * NEWS: Add entry documenting Ravenscar tasking support
        on AArch64 ELF.