rs6000: ICE when using an MMA type as a function param or return value [PR96506]

PR96506 shows a problem where we ICE on illegal usage, namely using MMA
types for function arguments and return values.  The solution is to flag
these illegal usages as errors early, before we ICE.

2020-08-13  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR target/96506
	* config/rs6000/rs6000-call.c (rs6000_promote_function_mode): Disallow
	MMA types as return values.
	(rs6000_function_arg): Disallow MMA types as function arguments.

gcc/testsuite/
	PR target/96506
	* gcc.target/powerpc/pr96506.c: New test.