[CMake] Unify install path for libraries

Introduce OPENMP_INSTALL_LIBDIR and use in all install() commands.
This also fixes installation of libomptarget-nvptx that previously
didn't honor {OPENMP,LLVM}_LIBDIR_SUFFIX.

Differential Revision: https://reviews.llvm.org/D47130

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@333284 91177308-0d34-0410-b5e6-96231b3b80d8