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.