[maccatalyst] Handle macCatalyst static swiftmodules (#76784)
The existing CMake code for macCatalyst swiftmodule only handled the
shared case, and did not have the static pieces that non-macCatalyst
targets are doing.
Try to match as much as possible the non-macCatalyst variables and
structure and allow macCatalyst targets to support static Swift targets.
This is important for the `Cxx.swiftmodule` and `CxxStdlib.swiftmodule`,
since they are only static, and the previous code skipped creating those
files since https://github.com/swiftlang/swift/pull/74994
