target/hexagon: Use float32_muladd_scalbn for helper_sffma_sc

target/hexagon: Use float32_muladd_scalbn for helper_sffma_sc

This instruction has a special case that 0 * x + c returns c
without the normal sign folding that comes with 0 + -0.
Use the new float_muladd_suppress_add_product_zero to
describe this.

Reviewed-by: Brian Cain <brian.cain@oss.qualcomm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>