[OpenMP][NVPTX] Added the missing -O1 when building NVPTX bitcode libraries

In the past `-O1` was used when building NVPTX bitcode libraries. After
we switched to OpenMP, `-O1` was missing by mistake, leading to a huge performance
regression.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D95545

(cherry picked from commit 5a64794b)