Many qemu softfloat will check floatx80_rounding_precision
even when berkeley testfloat will not.  So begin with
floatx80_precision_x, so that's the one we use
when !FUNC_EFF_ROUNDINGPRECISION.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>