[libomptarget-nvptx] reduction: Determine if runtime uninitialized

Pass in the correct value of isRuntimeUninitialized() which solves
parallel reductions as reported on the mailing list.
For reference: r333285 did the same for loop scheduling.

Differential Revision: https://reviews.llvm.org/D52725

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@343476 91177308-0d34-0410-b5e6-96231b3b80d8