Symbol export with llvm-ml

Hi all,

I thought I'd share 2 patches that I needed to successfully compile boost.context on clang-cl with llvm-ml. I think these can't exactly be applied as-is without breaking existing functionality, but it basically stems down to `/quiet` not being supported, and needing to pass `--m64` to enable 64-bit aka the `mt64.exe` equivalent. The other thing is how symbols get exported, the `PROC` call doesn't support `EXPORT` however `__declspec(dllexport)` on the C signature does work.

[cmake.patch](https://github.com/user-attachments/files/22249116/cmake.patch)

[fcontext.patch](https://github.com/user-attachments/files/22249117/fcontext.patch)

Kind regards