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.
It is causing bots to fail.
* Revert "The __has_include(<os/system_version.h>) branch here wasn't quite right, we'll just use the dlsym one for now"
This reverts commit f824922456.
* Revert "Remove stdlib and runtime dependencies on Foundation and CF"
This reverts commit 3fe46e3f16.
rdar://54709269