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.
LLVM doesn't have a stable ABI for Float16 on x86 yet; we're working with Intel to get that fixed, but we don't want to make the type available on macOS until a stable ABI is actually available, because we'd break binaries compiled before any calling convention changes if we do.
