Typify the arch used in toolchain selection

Summary: Per title, raise the string we're passing around to be a solid Arch type.

Reviewed By: rmaz

Differential Revision: D51481593

fbshipit-source-id: 867fa911dac4cfc22ed391b6c45a87e61267a0cf

prelude/platforms/apple/arch.bzl

@@ -0,0 +1,10 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under both the MIT license found in the
+# LICENSE-MIT file in the root directory of this source tree and the Apache
+# License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+# of this source tree.
+
+AppleArches = ["i386", "x86_64", "arm64", "arm64_32", "armv7k"]
+
+AppleArch = enum(*AppleArches)