[openmp] Unconditionally set march commandline argument

Extracted from D117246. This reflects the march value used by the
compile back into the toolchain arguments, letting downstream processes
such as LTO rely on it being present. Subsequent patches should also be able
to remove the two other calls to checkSystemForAMDGPU.

Reviewed By: jonchesterfield

Differential Revision: https://reviews.llvm.org/D117706