[lld] [lld] Add target support for SystemZ (s390x) (PR #75643)

[Fangrui Song via llvm-commits]

MaskRay wrote:

BTW, I wonder whether we should use `Arch/S390X.cpp` and `test/ELF/s390x-*`. There are precedents in llvm-project:

```
clang/test/Preprocessor/init-s390x.c
compiler-rt/lib/sanitizer_common/sanitizer_linux_s390.cpp
lldb/source/Plugins/ABI/SystemZ/ABISysV_s390x.cpp
```

https://github.com/llvm/llvm-project/pull/75643