Fix: expose std.math.ldexp as C ldexp, scalbn and scalbln in compiler-rt to address LLVM release-mode simplification

[th3james]

Updated version of #24006

Fixes #23358

Changes:

• Creates compiler-rt implementations that satisfy the libc symbols of the ldexp, scalbn and scalbln families, proxying the internal implementation to std.math.ldexp
  • scalbln family mirrors musl by clamping the long float
• Removes the musl and mingw implementations as they are now unused
• Adds an integration test for wasm-ld: ....o: undefined symbol: ldexp in Release modes, but works in Debug #23358

Impact:

• Standard Zig implementation of ldexp is used everywhere. Assembly optimised implementations are lost.

[alexrp]

Is this clamping behavior part of the C standard? We need to be careful about whether it should apply regardless of target.

[andrewrk]

Thanks @th3james.

Before this is merged:

1. the logic for these functions need to move from std into compiler_rt
2. new builtins need to be added for the functions
3. the std code needs to be deleted in favor of calling those builtin functions
4. new Legalize features added for expanding these builtin calls into compiler_rt calls
5. llvm backend disables them and emits intrinsic calls
6. other backends enable them

This is a pretty big scope increase, so you can hand it off to a core team member at any time.

[th3james]

Happy to hand this off to someone else, as a first issue this was already pushing the limits of my expertise and free time. I’ll leave my branch live in case it’s a useful point of reference, feel free to close this PR or convert it to an issue/draft as you see fit.
Thank you both for taking the time to review, I learned a tonne.