Fix math/test-fenv for no-exceptions / no-rounding-modes configurations.

This patch fixes math/test-fenv.c to check EXCEPTION_TESTS and
ROUNDING_TESTS to avoid failing in cases where some exceptions or
rounding modes are defined but not supported at runtime.

Tested for mips64 soft float and for x86_64.

	* math/test-fenv.c (fe_tests): Skip most tests when exceptions not
	supported.
	(feholdexcept_tests): Skip tests requiring exceptions or rounding
	modes support if not supported.