rs6000: Change optab for ibm128 and ieee128 conversionHEADtrunkmaster

Currently for 128 bit floating-point ibm128 and ieee128
formats conversion, the corresponding libcalls are:
  ibm128 -> ieee128 "__trunctfkf2"
  ieee128 -> ibm128 "__extendkftf2"
, and generic code handling (like convert_mode_scalar) also
adopts sext_optab for ieee128 -> ibm128 while trunc_optab
for ibm128 -> ieee128.  But in rs6000 port as function
rs6000_expand_float128_convert and init_float128_ieee show,
we adopt sext_optab for ibm128 -> ieee128 with "__trunctfkf2"
while trunc_optab for ieee128 -> ibm128 with "__extendkftf2".

To make them consistent and avoid some surprises, this patch
is to adjust rs6000 internal handlings by adopting trunc_optab
for ibm128 -> ieee128 with "__trunctfkf2" while sext_optab for
ieee128 -> ibm128 with "__extendkftf2".

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (init_float128_ieee): Use trunc_optab rather
	than sext_optab for converting FLOAT128_IBM_P mode to FLOAT128_IEEE_P
	mode, and use sext_optab rather than trunc_optab for converting
	FLOAT128_IEEE_P mode to FLOAT128_IBM_P mode.
	(rs6000_expand_float128_convert): Likewise.

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