cycles_util_avxf_avx2_test failed on the cross test, since
it wasn't immediately clear why, the test was disabled.
After looking into it, this test when build for AVX2 is
generating FMA instructions where the intermediate results
have "infinite" precision [1] leading to slightly different
results.
This diff re-enables the cross test and allows for a small error
in the results.
[1] https://www.felixcloutier.com/x86/vfmadd132ps:vfmadd213ps:vfmadd231ps
Differential Revision: https://developer.blender.org/D6873
Reviewers: brecht
