Complete tdep move to convert_typed_floating

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.

gdb/ChangeLog
2017-09-27  Ulrich Weigand  <uweigand@de.ibm.com>

	* arm-tdep.c: (convert_from_extended): Remove.
	(convert_to_extended): Likewise.
	(arm_extract_return_value): Use convert_typed_floating.
	(arm_store_return_value): Likewise.

	* sh-tdep.h (struct gdbarch_tdep): Add sh_littlebyte_bigword_type.
	* sh-tdep.c: Do not include "floatformat.h".
	(sh_littlebyte_bigword_type): New function.
	(sh_register_convert_to_virtual): Use convert_typed_floating.
	(sh_register_convert_to_raw): Likewise.
	* sh64-tdep.c: (struct gdbarch_tdep): Add sh_littlebyte_bigword_type.
	(sh64_littlebyte_bigword_type): New function.
	(sh64_extract_return_value): Use convert_typed_floating.
	(sh64_register_convert_to_virtual): Likewise.
	(sh64_register_convert_to_raw): Likewise.