arm/helper.c: re-factor rsqrte and add rsqrte_f16

arm/helper.c: re-factor rsqrte and add rsqrte_f16

Much like recpe the ARM ARM has simplified the pseudo code for the
calculation which is done on a fixed point 9 bit integer maths. So
while adding f16 we can also clean this up to be a little less heavy
on the floating point and just return the fractional part and leave
the calle's to do the final packing of the result.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180227143852.11175-27-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>