PowerPC: Add Decimal <-> float128 conversions.

PowerPC: Add Decimal <-> float128 conversions.

libgcc/
2020-11-11  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/t-float128 (fp128_dec_funcs): New macro.
	(ibm128_dec_funcs): New macro.
	(fp128_ppc_funcs): Add the Decimal <-> __float128 conversions.
	(fp128_dec_objs): Force Decimal <-> __float128 conversions to be
	compiled with -mabi=ieeelongdouble.
	(ibm128_dec_objs): Force Decimal <-> __float128 conversions to be
	compiled with -mabi=ieeelongdouble.
	(FP128_CFLAGS_DECIMAL): New macro.
	(IBM128_CFLAGS_DECIMAL): New macro.
	* config/rs6000/_dd_to_kf.c: New file.
	* config/rs6000/_kf_to_dd.c: New file.
	* config/rs6000/_kf_to_sd.c: New file.
	* config/rs6000/_kf_to_td.c: New file.
	* config/rs6000/_sd_to_kf.c: New file.
	* config/rs6000/_td_to_kf.c: New file.