Add libm benchmarks

[joanaxcruz]

Checklist

• I have read the contributing guidelines.
• I have considered portability of my change across platforms and architectures.
• I have self-reviewed my code.
• I have commented my code where necessary.
• I have updated the documentation accordingly.
• I have added tests that prove my fix is effective or that my feature works.

What is the purpose of this pull request?

• Improve benchmark tool
• Documentation update

What changes did you make?

Extended current benchmark tool with support to benchmark of libm functions. Libm benchmark is enabled in all functions listed in benchlibm.cpp.
It interfaces with the rest of the existing tool through benchmark_callers_libm.hpp (defines macros which simplify how the list of functions to benchmark looks like as it groups functions by precision/accuracy/variant).
The tool is integrated with SLEEF via CMake, meaning it can be built automatically when SLEEF is built. To enable that, pass CMake argument -DSLEEF_BUILD_BENCH=ON and
-DSLEEF_BUILD_BENCH_REF=ON.
Note this option is only enabled on Linux OS.

Is there anything you would like reviewers to focus on?

Subject to internal review