Many tdep files need to perform conversions between two floating-point
types, usually when accessing FP registers. Most targets now use the
convert_typed_floating helper routine to do so. However, a small number
still use the old method of converting via a DOUBLEST. Since we want
to get rid of DOUBLEST, these targets need to be moved to the new
method as well.
The main obstacle is that for convert_typed_floating we need an actual
*type*, not just a floatformat.
In arm-tdep.c, this is very straightforward, since there is already a
type using the ARM extended floatformat.
For sh-tdep.c and sh64-tdep.c, no such type already exists, so I've
added one to the gdbarch_tdep struct as done on other targets.