Add basic support for AArch64.

        * Makefile.in (ALL_64_TARGET_OBS): Add arch64-tdep.o.
        (HFILES_NO_SRCDIR): Add aarch64-tdep.h.
        (ALLDEPFILES): Add aarch64-tdep.c.
        * aarch64-tdep.c: New file.
        * aarch64-tdep.h: New file.
        * configure.tgt: Add AArch64.
        * features/Makefile (WHICH): Add aarch64 and aarch64-without-fpu.
        (aarch64-expedite): New definition.
        * features/aarch64-core.xml: New file.
        * features/aarch64-fpu.xml: New file.
        * features/aarch64-without-fpu.c: New file (generated).
        * features/aarch64-without-fpu.xml: New file.
        * features/aarch64.c: New file (generated).
        * features/aarch64.xml: New file.
        * regformats/aarch64-without-fpu.dat: New file (generated).
        * regformats/aarch64.dat: New file (generated).