testsuite: arm: tighten up mode-specific ISA tests

Some of the standard Arm architecture tests require the test to use a
specific instruction set (arm or thumb).  But although the framework
was checking that the flag was accepted, it wasn't checking that the
flag wasn't somehow being override (eg by run-specific options).  We
can improve these tests easily by checking whether or not __thumb-_ is
defined.

gcc/testsuite:

	* lib/target-supports.exp (check_effective_target_arm_arch_FUNC_ok):
	For instruction-set specific tests, check that __thumb__ is, or
	isn't defined as appropriate.