[openmp] No longer use LIBRARY_PATH to find devicertl

Given D109057, change test runner to use the libomptarget-x-bc-path
argument instead of the LIBRARY_PATH environment variable to find the device
library.

Also drop the use of LIBRARY_PATH environment variable as it is far
too easy to pull in the device library from an unrelated toolchain by accident
with the current setup. No loss in flexibility to developers as the clang
commandline used here is still available.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D109061