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.