math: support platforms with limited FP rounding or exception support

math: support platforms with limited FP rounding or exception support

For some tests, just claim that fetestexcept() always returns true,
so the rest of the test can be compiled.

For libm-test, provide known bogus values for unsupported rounding
modes, so fesetround() will return failure.

Elsewhere, just add some #ifdefs to avoid code that uses particular
FP exceptions if the exceptions aren't supported.