[Driver] Corrections for linker flags passed with relocatable linking on OpenBSD (#67254)

The entry point symbol handling matches our GCC link spec..
```%{!shared:%{!nostdlib:%{!r:%{!e*:-e __start}}}}```

Remove usage of -Bdynamic as it is the default for the linker anyway.

Came up in discussion here https://github.com/llvm/llvm-project/pull/65644