testsuite, Darwin : Do not claim 'GAS' for cctools assembler.

Although the cctools assembler is based of GNU GAS, it is from a
very old version (1.38) which does not support many of the features
that the target supports test is expecting.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp: Exclude cctools assembler based on
	GAS 1.38.