asin(x) is now implemented using a piecewise approximation, which
improves the precision for |x| < 0.5
Previously, we were using a polynomial approximation for both the
asin() and acos() functions. Unfortunately, for asin(), this polynomial
does not have enough precision to satisfy the Vulkan CTS requiremenents,
which define the asin() precision based on the precision of
atan2(x, sqrt(1.0 - x*x)). The piecewise approximation gives the needed
precision in the problematic range.

v2: Skip the piecewise approximation for acos

Closes: #1843

Acked-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <mesa!3809>