[Libomptarget] Fix multiply defined symbol during linking

This patch adds the `weak` identifier to the openmp device environment
variable. The changes introduced in https://reviews.llvm.org/D117211
result in multiply defined symbols. Because the symbol is potentially
included multiple times for each offloading file we will get symbol
colisions, and because it needs to have external visiblity it should be
weak.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D117231