Remove the use of Base.libm_name

[ViralBShah]

We'll be removing Base.libm_name in Julia 1.9, and eventually remove OpenLibm from base.

JuliaLang/julia#42299

To continue using OpenLibm, simply use OpenLibm_jll explicitly in your Project.toml, and this recipe should allow you to keep using libm_name as before:

using OpenLibm_jll
libm_name = OpenLibm_jll.libopenlibm

If you are using libm_name mainly for informational purposes, it can be deleted, since Julia uses its native implementation for most libm functions, and will default to system libm when necessary.