build: remove openlibm

[inkydragon]

This pr replaced #42299

On Windows, we use libmingwex to provide math functions which are not implemented by msvcrt (Old Microsoft Libm).
From libmingwex: ldexp, log2l, exp2l, powi

Note: We are using a special toolchain i686-12.2.0-release-posix-sjlj from niXman/mingw-builds-binaries for i686-windows.

[inkydragon]

do not merge this pr.

[inkydragon]

With newer msvcrt from gcc14, we can build julia-x64 without openblibm
https://buildkite.com/julialang/julia-master/builds/43184#0193e4ca-544e-43fd-b419-b3c2d245ba39

[inkydragon]

Now, we passed all tests on x86 and x64

https://buildkite.com/julialang/julia-master/builds/43191#0193e8a8-9d95-47cb-95e9-403619dd44ed

[giordano]

With newer msvcrt from gcc14, we can build julia-x64 without openblibm

Does this mean that on Windows we'd need to require a very recent compiler?

[inkydragon]

Does this mean that on Windows we'd need to require a very recent compiler?

MSYS2 only keeps the latest version of the package, so GCC 14 is used here.

Maybe a newer version of gcc isn't necessary, but I haven't tested it yet.
I guess just using the mingw toolchain and linking to mingw32, mingwex, msvcrt would be an alternative to openlibm.

[oscardssmith]

As an alternative to this, could we just use LLVM libm? It's not complete, but IIRC the only thing that we actually depend on is the conversions which are implemented https://libc.llvm.org/headers/math/index.html#implementation-status

[inkydragon]

As an alternative to this

After this PR, we use system libm.
Then, you can switch to LLVM libm, if you like.
Or, we can provide the configuration for switching libm in the build system.