rs6000: Fix ICE in rs6000_init_builtins when compiling with -mcpu=440 [PR99279]

rs6000: Fix ICE in rs6000_init_builtins when compiling with -mcpu=440 [PR99279]

The initialization of compat builtins assumes the builtin we are creating
a compatible builtin for exists and ICEs if it doesn't.  However, there are
valid reasons why some builtins are disabled for a particular compile.
In this case, the MMA builtins are disabled for -mcpu=440 (and other cpus),
so instead of ICEing, we should just skip adding the MMA compat builtin.

2021-02-25  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR target/99279
	* config/rs6000/rs6000-call.c (rs6000_init_builtins): Replace assert
	with an "if" test.