Commit the platform definition and build script work necessary to
cross-compile for arm64_32.
arm64_32 is a variant of AARCH64 that supports an ILP32 architecture.
JSONEcoder/Decoder get another underscore in their ObjC name, as the one with a single underscore conflicts with older Swifts that had _JSONEcoder/Decoder as internal rather than fileprivate.
_JSONEncoder/Decoder get another underscore in their Swift name to avoid conflicting with the ObjC name from the ones in older overlays.
Turns out some people used this type despite it being prefixed with
`_stdlib_`, so we have to keep it, with an obsoletion message this time.
Second copy of the same type is kept available past Swift 5 in
SwiftPrivate for use in tests.
