Update sleef functions

[Thanatomanic]

Somewhere in the past we have adopted functions from SLEEF, see https://github.com/shibatch/sleef. However, I noticed recently that we use a very old version, probably 2.90 dated April 2016. In the meanwhile several SLEEF functions have had improvements (probably to do with either speed or accuracy), but those were never ported to RT.
The latest release in the 2.# branch is 2.121 (dated Jan 30, 2017), and then a new major branch with breaking changes was started and continued. The latest release in the 3.# branch is 3.5.1 (dated Sep 15, 2020).

Unless somebody (hopefully @heckflosse or @Floessie) tells me it is unnecessary, I would like to continue to update our SLEEF functions to their current optimal forms. First I will bring them up to date with the 2.121 version and potentially continue making changes to adopt the 3.5.1 version.

How does that sound?

So far I have made a round of updates for sleef.h only which you can give a testrun. It compiles and links, so I guess I haven't already broken something. I haven't touched the SSE2 functions.

[Thanatomanic]

To clarify: if you look at the changed files you can see I removed some functions. These were unused in the RT source code. I kept double precision functions even if they were unused because I think they ought to be the functions you preferentially choose when developing new code.