[compiler-rt] [builtins][CMake] Replace custom target for lse_builtin symlinks (PR #66936)

[Shoaib Meenai via llvm-commits]

https://github.com/smeenai approved this pull request.

Never mind, I looked at the whole file and understand this better now. We're still adding the generated files to the sources. I thought there might be issues doing that with non-Ninja generators, but https://cmake.org/cmake/help/latest/command/add_custom_command.html#examples-generating-files seems to explicitly bless it, so we should be good.

The only thing that stood out to me on that same page was:
> Each source file may have at most one command specifying it as its main dependency.

Which we're violating. Dunno if it matters in practice, but we could just use `DEPENDS` instead?

https://github.com/llvm/llvm-project/pull/66936