[OpenMP] Make offloading sections have the SHF_EXCLUDE flag

Offloading sections can be embedded in the host during codegen via a
section. This section was originally marked as metadata to prevent it
from being loaded, but these sections are completely unused at runtime
so the linker should automatically drop them from the final executable
or shard library. This flag adds support for the SHF_EXCLUDE flag in
target lowering and uses it.

Reviewed By: JonChesterfield, MaskRay

Differential Revision: https://reviews.llvm.org/D122987