[OpenMP] Ensure offloading sections do not have SHF_ALLOC flag

We use offloading sections in the new Clang driver scheme to embed
device code into the host. We later use these sections to link the
device image, after which point they are completely unused and should
not be loaded into memory if they are still in the executable.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D120275