This adds a new copy-legacy-layouts-${platform}-${arch} target for each
platform and architecture that the standard library is built for.
If the platform and architecture has a corresponding layout file in
stdlib/public/legacy_layouts/${platform}/layouts-${arch}.yaml, the
target copies this file to the build directory; otherwise, it does
nothing.
When building Swift code, the subroutines in SwiftSource.cmake add a
dependency on this target from each Swift code target.
Finally, we ensure that the YAML files are copied into the toolchain
package when building a toolchain.
