Add the aarch64-apple-darwin target

[shepmaster]

This is a basic copy-paste-modify from the existing
x86_64-apple-darwin target.

[rust-highfive]

r? @varkor

(rust_highfive has picked a reviewer for you, use r? to override)

[shepmaster]

This was copied from LLVM.

[shepmaster]

% cat size.c
#include <stdio.h>

int main() {
  printf("%zu\n", sizeof(int));
  return 0;
}

% cc size.c

% file a.out
a.out: Mach-O 64-bit executable arm64

% ./a.out
4

[jyn514]

You might also want printf("%zu %zu\n", sizeof(int*), sizeof(int (*)()));. Not that I expect it to be anything other than 8 bytes. Whoops, that was your next comment!

[shepmaster]

% cat size.c
#include <stdio.h>

int main() {
  printf("%zu\n", sizeof(void *));
  return 0;
}

% cc size.c

% file a.out
a.out: Mach-O 64-bit executable arm64

% ./a.out
8

[shepmaster]

These additions allow cross-compiling from aarch64 back to x86_64.

[nagisa]

@bors r+

[bors]

📌 Commit 804241e has been approved by nagisa

[spastorino]

Adding missing T-compiler label so it can be automatically picked up and placed in the weekly agenda.

[pnkfelix]

discussed in T-compiler meeting. beta-accepted.

[Mark-Simulacrum]

Note that we only cherry-picked the first commit here into beta, not the Cargo.lock changes.