Simplify floatformat_from_type

For historical reasons, the TYPE_FLOATFORMAT element is still set to hold
an array of two floatformat structs, one for big-endian and the other for
little-endian.  When accessing the element via floatformat_from_type,
the code would check the type's byte order and return the appropriate
floatformat.

However, these days this is quite unnecessary, since the type's byte order
is already known at the time the type is allocated and the floatformat is
installed into TYPE_FLOATFORMAT.  Therefore, we can just install the correct
version here.

Also, moves the (now trivially simple) floatformat_from_type accessor to
gdbtypes.{c,h}, since it doesn't really need to be in doublest.c now.

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

	* doublest.h (floatformat_from_type): Move to gdbtypes.h.
	* doublest.c (floatformat_from_type): Move to gdbtypes.c.

	* gdbtypes.h (union type_specific): Make field floatformat hold
	just a single struct floatformat, not an array.
	(floatformat_from_type): Move here.
	* gdbtypes.c (floatformat_from_type): Move here.  Update to
	changed TYPE_FLOATFORMAT definition.
	(verify_floatformat): Update to changed TYPE_FLOATFORMAT.
	(recursive_dump_type): Likewise.
	(init_float_type): Install correct floatformat for byte order.
	(arch_float_type): Likewise.