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

[Fangrui Song via llvm-commits]

MaskRay wrote:

This option is relatively obscure but offers value when necessary.
The description of its supported linker script syntax is in `docs/ELF/linker_script.rst `

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