[lld] [lld][ELF]Emit warning when both scripts are specified (PR #163497)

[Mingming Liu via llvm-commits]

mingmingl-llvm wrote:

> 
One thought is whether we should be documenting --default-script for users at all, if it's effectively a hidden option for toolchains to use. Maybe not making it visible to users might reduce confusion?

I'm in favor of the idea to make `--default-script` more visible to toolchain users. Is there a canonical (`.rst`) file to make this change? Or is it more conventional to start a new `.rst` for linker script support, like https://lld.llvm.org/ELF/start-stop-gc.html does for 'start-stop-gc'?

https://github.com/llvm/llvm-project/pull/163497