[OpenMP][FIX] Properly align firstprivate variables

The old code didn't actually align the values, and it added padding even
when none was necessary. This approach will pad entries if necessary
and, similar to the struct case, use the host pointer as guidance.

NOTE: This does still not align them as the host has, but it's unclear
      if the user really should use the alignment bits anyway. For now
      this is a reasonable compromise, only if we have host alignment
      information (explicitly not implicitly via the host pointer), we
      could do it completely right without wasting lots of resources for
      >99% of the cases.

Fixes: https://github.com/llvm/llvm-project/issues/61034