[libomptarget][cuda] Handle missing _v2 symbols gracefully

[libomptarget][cuda] Handle missing _v2 symbols gracefully

Follow on from D95367. Dlsym the _v2 symbols if present, otherwise use the
unsuffixed version. Builds a hashtable for the check, can revise for zero
heap allocations later if necessary.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D95415