PowerPC: Improve ibm/ieee 128-bit error messages.

PowerPC: Improve ibm/ieee 128-bit error messages.

gcc/
2020-09-15  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/rs6000.c (rs6000_invalid_binary_op): Update error
	messages about mixing IBM long double and IEEE 128-bit.

gcc/testsuite/
2020-09-15  Michael Meissner  <meissner@linux.ibm.com>

	* gcc.target/powerpc/bfp/scalar-extract-exp-4.c: Update failure
	messages.
	* gcc.target/powerpc/bfp/scalar-extract-sig-4.c: Update failure
	messages.
	* gcc.target/powerpc/bfp/scalar-test-data-class-11.c: Update
	failure messages.
	* gcc.target/powerpc/bfp/scalar-test-neg-5.c: Update failure
	messages.
	* 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.