FPU: Implement fsqrt[s] and add a test for fsqrt

FPU: Implement fsqrt[s] and add a test for fsqrt

This implements the floating square-root calculation using a table
lookup of the inverse square root approximation, followed by three
iterations of Goldschmidt's algorithm, which gives estimates of both
sqrt(FRB) and 1/sqrt(FRB).  Then the residual is calculated as
FRB - R * R and that is multiplied by the 1/sqrt(FRB) estimate to get
an adjustment to R.  The residual and the adjustment can be negative,
and since we have an unsigned multiplier, the upper bits can be wrong.
In practice the adjustment fits into an 8-bit signed value, and the
bottom 8 bits of the adjustment product are correct, so we sign-extend
them, divide by 4 (because R is in 10.54 format) and add them to R.

Finally the residual is calculated again and compared to 2*R+1 to see
if a final increment is needed.  Then the result is rounded and
written back.

This implements fsqrts as fsqrt, but with rounding to single precision
and underflow/overflow calculation using the single-precision exponent
range.  This could be optimized later.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>