target/openrisc: Implement unordered fp comparisons

These were added to the 1.3 spec.  For OF32S, validate AVR.
But OF64A32 is itself new to 1.3 so no extra check needed.

Reviewed-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>