[OpenMP] Allow compiling multiple target architectures with OpenMP

This patch adds support for OpenMP to use the `--offload-arch` and
`--no-offload-arch` options. Traditionally, OpenMP has only supported
compiling for a single architecture via the `-Xopenmp-target` option.
Now we can pass in a bound architecture and use that if given, otherwise
we default to the value of the `-march` option as before.

Note that this only applies the basic support, the OpenMP target runtime
does not yet know how to choose between multiple architectures.
Additionally other parts of the offloading toolchain (e.g. LTO) require
the `-march` option, these should be worked out later.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D124721