Add the aarch64-apple-darwin target

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

Author: shepmaster

src/librustc_target/spec/aarch64_apple_darwin.rs

@@ -0,0 +1,30 @@
+use crate::spec::{LinkerFlavor, Target, TargetOptions, TargetResult};
+
+pub fn target() -> TargetResult {
+    let mut base = super::apple_base::opts();
+    base.cpu = "apple-a12".to_string();
+    base.max_atomic_width = Some(128);
+    base.pre_link_args.insert(LinkerFlavor::Gcc, vec!["-arch".to_string(), "arm64".to_string()]);
+
+    base.link_env_remove.extend(super::apple_base::macos_link_env_remove());
+
+    // Clang automatically chooses a more specific target based on
+    // MACOSX_DEPLOYMENT_TARGET.  To enable cross-language LTO to work
+    // correctly, we do too.
+    let arch = "aarch64";
+    let llvm_target = super::apple_base::macos_llvm_target(&arch);
+
+    Ok(Target {
+        llvm_target,
+        target_endian: "little".to_string(),
+        target_pointer_width: "64".to_string(),
+        target_c_int_width: "32".to_string(),
+        data_layout: "e-m:o-i64:64-i128:128-n32:64-S128".to_string(),
+        arch: arch.to_string(),
+        target_os: "macos".to_string(),
+        target_env: String::new(),
+        target_vendor: "apple".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
+        options: TargetOptions { target_mcount: "\u{1}mcount".to_string(), ..base },
+    })
+}

src/librustc_target/spec/mod.rs

@@ -574,6 +574,7 @@ supported_targets! {
     ("i686-unknown-haiku", i686_unknown_haiku),
     ("x86_64-unknown-haiku", x86_64_unknown_haiku),
 
+    ("aarch64-apple-darwin", aarch64_apple_darwin),
     ("x86_64-apple-darwin", x86_64_apple_darwin),
     ("i686-apple-darwin", i686_apple_darwin),
 

src/librustc_target/spec/x86_64_apple_darwin.rs

@@ -5,7 +5,10 @@ pub fn target() -> TargetResult {
     base.cpu = "core2".to_string();
     base.max_atomic_width = Some(128); // core2 support cmpxchg16b
     base.eliminate_frame_pointer = false;
-    base.pre_link_args.insert(LinkerFlavor::Gcc, vec!["-m64".to_string()]);
+    base.pre_link_args.insert(
+        LinkerFlavor::Gcc,
+        vec!["-m64".to_string(), "-arch".to_string(), "x86_64".to_string()],
+    );
     base.link_env_remove.extend(super::apple_base::macos_link_env_remove());
     base.stack_probes = true;