PR23877, bad value (n32r5900) for default CPU

Catching this at configure time would be nicer, but we'd need to exactly
match mips_parse_cpu in configure.ac and keep it all in sync.

	PR 23877
	* config/tc-mips.c (mips_after_parse_args): Don't assert that
	mips_parse_cpu returns non-NULL, call as_fatal with an informative
	message instead.