Add support for ELF core files on OpenBSD/sparc64.

gdb/ChangeLog:

        * sparc64obsd-tdep.c (sparc64obsd_gregset): New variable.
        (sparc64obsd_supply_gregset): Handle registers sets used in ELF
        core dumps.
        (sparc64obsd_init_abi): Adjust minimum size of the general purpose
        register set used in ELF core dumps.  Add floating-point register set.