[Libomptarget] Replace std::vector with llvm::SmallVector

The runtime makes some use of `std::vector` data structures. We should
be able to replace these trivially with `llvm::SmallVector` instead.
This should allow us to avoid heap allocations in the majority of cases
now.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D130927