Add ravenscar-thread support for powerpc.

Add ravenscar-thread support for powerpc.

gdb/ChangeLog:

        * ppc-ravenscar-thread.h, ppc-ravenscar-thread.c: New files.
        * Makefile.in (ALL_TARGET_OBS): Add ppc-ravenscar-thread.o.
        (HFILES_NO_SRCDIR): Add ppc-ravenscar-thread.h.
        (ALLDEPFILES): Add ppc-ravenscar-thread.c.
        * configure.tgt: Add ravenscar-thread.o and ppc-ravenscar-thread.o
        to gdb_target_obs for every target that uses rs6000-tdep.o.
        * rs6000-tdep.c: #include "ppc-ravenscar-thread.h".
        (rs6000_gdbarch_init): Call register_e500_ravenscar_ops
        or register_ppc_ravenscar_ops.