Make fallback fesetexceptflag always succeed (bug 21028).

The fallback implementation of fesetexceptflag currently fails if any
exceptions are specified.  It should always succeed, because the
exception state is always that all exceptions (if any are defined in
<fenv.h> but not supported in this configuration) are always clear,
just as fallback fetestexcept always succeeds and fallback fesetenv
always succeeds unless asked to set FE_NOMASK_ENV.

This patch fixes it accordingly.  Together with the patch to
test-fexcept.c to allow feraiseexcept to fail in another place, this
stops that test from failing for MIPS soft-float.

Tested for mips64 soft-float.

	[BZ #21028]
	* math/fsetexcptflg.c (__fesetexceptflag): Always return 0.
	* math/test-fexcept.c (test_set): Allow failure of feraiseexcept
	if EXCEPTION_TESTS returns false.