[ia64] Add a big-endian version of the ia64-ext floatformat

[ia64] Add a big-endian version of the ia64-ext floatformat

ia64-tdep.c defines a floatformats_ia64_ext that should contain
both the little-endian and the big-endian version of the float
format used in the ia64 registers (an 82bit float format).
Right now, both entries point to the same little-endian definition.

A big-endian definition is now necessary for the ia64-hpux port.

gdb/ChangeLog:

        * ia64-tdep.c (floatformat_ia64_ext_little): Renames
        floatformat_ia64_ext.
        (floatformat_ia64_ext_big): New static const.
        (floatformats_ia64_ext): Set first entry to &floatformat_ia64_ext_big.