powerpc: use float64 for frsqrte

Remove the code that reduce the result to float32 as the frsqrte
instruction is defined to return a double-precision estimate of
the reciprocal square root.

Although reducing the fractional part is harmless (as the estimation
must have at least 12 bits of precision according to the old PEM),
reducing the exponent range is not correct.

Signed-off-by: Tristan Gingold <gingold@adacore.com>
Signed-off-by: Alexander Graf <agraf@suse.de>