Cycles: Use fast math functions in hair BSDF

Precision of the fast functions seems to be enough in there and
since the code was heavily using inverse trigonometric functions
this change gives few percent speedup on Victor's hair.

From the tests files from ctests storage doesn't have any meaningful
difference, hair on Victor is all below 4% absolute error and only
few pixels are exceeding 1% absolute difference.

In any case, let it be as it is currently so it allows us to have
fast math file in sources for it's further evaluation and possible
usage in other areas as well.

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