power-ieee128: Fix up byte-swapping for IBM extended real(kind=16)

Here is a patch to fix up the ppc64be vs. ppc64le byteswapping
of IBM extended real(kind=16) and complex(kind=16).
Similarly to the BT_COMPLEX case it halves size and doubles nelems
for the bswap_array calls.  Of course for r16_ibm and r16_ieee conversions
one needs to make sure it is only done when the on file data is in that
format and not in IEEE quad.

2022-01-11  Jakub Jelinek  <jakub@redhat.com>

	* io/transfer.c (unformatted_read, unformatted_write): When
	byteswapping IBM extended real(kind=16), handle it as byteswapping
	two real(kind=8) values.