testsuite, x86: Handle a broken assembler

Earlier assembler support for complex fp16 on x86_64 Darwin is broken.
This adds an additional test to the existing target-supports that fails
for the broken assemblers but works for the newer, fixed, ones.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp: Test an asm line that fails on broken
	Darwin assembler versions.