[OpenMP][OMPT] Fix device identifier collision during callbacks (#65595)

Fixes: https://github.com/llvm/llvm-project/issues/65104
When a user assigns devices to target regions it may happen that
different identifiers will map onto the same id within different
plugins. This will lead to situations where callbacks will become much
harder to read, as ambiguous identifiers are reported.

We fix this by collecting the index-offset upon general RTL
initialization. Which in turn, allows to calculate the unique,
user-observable device id.