target/ppc: Update fres to new flags and float64r32

target/ppc: Update fres to new flags and float64r32

There is no double-rounding bug here, because the result is
merely an estimate to within 1 part in 256, but perform the
operation with float64r32_div for consistency.

Use float_flag_invalid_snan instead of recomputing the
snan-ness of the operand.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211119160502.17432-34-richard.henderson@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>