[lld] [ELF] Rename target-specific RelExpr enumerators (PR #118424)

[Sam Elliott via llvm-commits]

================
@@ -90,41 +90,41 @@ enum RelExpr {
   // Even though RelExpr is intended to be a target-neutral representation
   // of a relocation type, there are some relocations whose semantics are
   // unique to a target. Such relocation are marked with R_<TARGET_NAME>.
----------------
lenary wrote:

```suggestion
  // unique to a target. Such relocation are marked with RE_<TARGET_NAME>.
```

https://github.com/llvm/llvm-project/pull/118424