[runtimes][openmp] Change to not treat ARCH-unknown-linux-gnu as errors

When OpenMP is compiled as a part runtimes for multiple targets, openmp
is compiled under build/runtimes/runtimes-arch-unknown-linux-gnu-bins
directory.  Old implementation treats this directory name as errors.
This patch adds a guard like "[Uu]known[^-]".

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D114346