rs6000: Fix ICE for invalid constants in built-in functions

For invalid constant operand values used in built-in functions, return
const0_rtx to signify an error occurred during expansion.

2025-01-16  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	* config/rs6000/rs6000-builtin.cc (rs6000_expand_builtin): Return
	const0_rtx when there is an error.

gcc/testsuite/
	* gcc.target/powerpc/mma-builtin-error.c: New test.