PowerPC: Adjust float128/ibm128 warnings.

PowerPC: Adjust float128/ibm128 warnings.

This patch ccombines two patches:

1) If GLIBC is 2.32 or newer, and the language is C or C++, allow the user to
   change the long double type without having to use -Wno-psabi.

2) Adjust the warnings for intermixing __float128 and __ibm128 to accomidate
   the future change to allow long double to use the IEEE 128-bit format.

gcc/
2020-10-29  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/rs6000.c (rs6000_option_override_internal): Allow
	long double type to be changed for C/C++ if glibc 2.32 or newer.
	(rs6000_invalid_binary_op): Update error messages about mixing IBM
	long double and IEEE 128-bit.

gcc/testsuite/
2020-10-27  Michael Meissner  <meissner@linux.ibm.com>

	* gcc.target/powerpc/float128-mix-2.c: New test.
	* gcc.target/powerpc/float128-mix-3.c: New test.
	* gcc.target/powerpc/float128-mix.c: Update failure messages.