Power10: Add missing IEEE 128-bit XSCMP* built-in mappings.

This patch adds support for mapping the scalar_cmp_exp_qp_* built-in functions
to handle arguments that are either TFmode or KFmode, depending on whether long
double uses the IEEE 128-bit representation (TFmode) or the IBM 128-bit
representation (KFmode).  This shows up in the float128-cmp2-runnable.c test
when long double uses the IEEE 128-bit representation.

gcc/
2020-11-20  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/rs6000-call.c (rs6000_expand_builtin): Add missing
	XSCMP* cases for IEEE 128-bit long double.