softfloat: Use post test for floatN_mul

softfloat: Use post test for floatN_mul

The existing f{32,64}_addsub_post test, which checks for zero
inputs, is identical to f{32,64}_mul_fast_test.  Which means
we can eliminate the fast_test/fast_op hooks in favor of
reusing the same post hook.

This means we have one fewer test along the fast path for multiply.

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