Add low accuracy sin, cos, log2, exp2 and pow

[shibatch]

I added the following functions.

• fastsinf_u3500
• fastcosf_u3500
• fastpowf_u3500
• log2_u35, log2f_u35
• exp2_u35, exp2f_u35

The error bound of fastsin and fastcos are max(2e-6, 350ULPs).
Each function has corresponding deterministic functions.

@xoofx Please let me know how you think about these functions.

[xoofx]

Amazing, will have a look, thanks!

[shibatch]

Testing at Travis is failing because brew is trying to build gcc from its source code.

[fpetrogalli]

Why introduce the fast prefix in the names?

[shibatch]

Because the error bound is not only specified by ULP value.
The fast trig functions sometimes returns values higher than 1.
The pow function tends to underflow easily.
So, they are pretty sloppy functions.

[fpetrogalli]

Testing at Travis is failing because brew is trying to build gcc from its source code.

Is this something that is broken also on master? If so, I think we should fix master first...

[shibatch]

I am hoping that this "problem" will be fixed at brew side.
Otherwise, we need to change the CI setting, of course.