[openmp] Don't copy exports into the source folder by default.

Additionally fix the copy if enabled on multi-config targets.

Summary:
This changes the copy command for libomp.so to use the output of the target as
the source of the copy, rather than trying to find it based on
${LIBOMP_LIBRARY_DIR}, which appears to be incorrect in multi-config generator
builds.

Reviewers: jdoerfert

Subscribers: mgorny, yaxunl, guansong, sstefan1, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D84148