Create aarch64-apple-darwin.md

Author: TimTheBig

src/doc/rustc/src/platform-support/aarch64-apple-darwin.md

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+# `aarch64-apple-darwin`
+
+**Tier: 2 with host tools**
+
+One-sentence description of the target (e.g. CPU, OS)
+
+## Target maintainers
+
+- ???
+
+## Requirements
+
+Does the target support host tools, or only cross-compilation? Does the target
+support std, or alloc (either with a default allocator, or if the user supplies
+an allocator)?
+
+Document the expectations of binaries built for the target. Do they assume
+specific minimum features beyond the baseline of the CPU/environment/etc? What
+version of the OS or environment do they expect?
+
+Are there notable `#[target_feature(...)]` or `-C target-feature=` values that
+programs may wish to use?
+
+What calling convention does `extern "C"` use on the target?
+
+What format do binaries use by default? ELF, PE, something else?
+
+## Building the target
+
+If Rust doesn't build the target by default, how can users build it? Can users
+just add it to the `target` list in `config.toml`?
+
+## Building Rust programs
+
+Rust does not yet ship pre-compiled artifacts for this target. To compile for
+this target, you will either need to build Rust with the target enabled (see
+"Building the target" above), or build your own copy of `core` by using
+`build-std` or similar.
+
+## Testing
+
+Does the target support running binaries, or do binaries have varying
+expectations that prevent having a standard way to run them? If users can run
+binaries, can they do so in some common emulator, or do they need native
+hardware? Does the target support running the Rust testsuite?
+
+## Cross-compilation toolchains and C code
+
+Does the target support C code? If so, what toolchain target should users use
+to build compatible C code? (This may match the target triple, or it may be a
+toolchain for a different target triple, potentially with specific options or
+caveats.)