Start adding an `aarch64-pc-windows-msvc` target

This commit adds the necessary definitions for target specs and such as well as
the necessary support in libstd to compile basic `aarch64-pc-windows-msvc`
binaries. The target is not currently built on CI, but it can be built locally
with:

    ./configure --target=aarch64-pc-windows-msvc --set rust.lld
    ./x.py build src/libstd --target aarch64-pc-windows-msvc

Currently this fails to build `libtest` due to a linker bug (seemingly in LLD?)
which hasn't been investigate yet. Otherwise though with libstd you can build a
hello world program (linked with LLD). I've not tried to execute it yet, but it
at least links!

Full support for this target is still a long road ahead, but this is hopefully a
good stepping stone to get started.

Points of note about this target are:

* Currently defaults to `panic=abort` as support is still landing in LLVM for
  SEH on AArch64.
* Currently defaults to LLD as a linker as I was able to get farther with it
  than I was with `link.exe`

3 files changed, 71 insertions, 29 deletions

diff --git a/src/libpanic_unwind/wasm32.rs b/src/libpanic_unwind/dummy.rs
index 7150560b4a1..7150560b4a1 100644
--- a/src/libpanic_unwind/wasm32.rs
+++ b/src/libpanic_unwind/dummy.rs
diff --git a/src/libpanic_unwind/lib.rs b/src/libpanic_unwind/lib.rs
index a61b2c1f063..9c3fc76c307 100644
--- a/src/libpanic_unwind/lib.rs
+++ b/src/libpanic_unwind/lib.rs
@@ -55,36 +55,33 @@ use core::mem;
 use core::raw;
 use core::panic::BoxMeUp;
 
-// Rust runtime's startup objects depend on these symbols, so make them public.
-#[cfg(all(target_os="windows", target_arch = "x86", target_env="gnu"))]
-pub use imp::eh_frame_registry::*;
+#[macro_use]
+mod macros;
 
-// *-pc-windows-msvc
-#[cfg(target_env = "msvc")]
-#[path = "seh.rs"]
-mod imp;
-
-// x86_64-pc-windows-gnu
-#[cfg(all(windows, target_arch = "x86_64", target_env = "gnu"))]
-#[path = "seh64_gnu.rs"]
-mod imp;
-
-// i686-pc-windows-gnu and all others
-#[cfg(any(all(unix, not(target_os = "emscripten")),
-          target_os = "cloudabi",
-          target_os = "redox",
-          all(windows, target_arch = "x86", target_env = "gnu")))]
-#[path = "gcc.rs"]
-mod imp;
-
-// emscripten
-#[cfg(target_os = "emscripten")]
-#[path = "emcc.rs"]
-mod imp;
-
-#[cfg(all(target_arch = "wasm32", not(target_os = "emscripten")))]
-#[path = "wasm32.rs"]
-mod imp;
+cfg_if! {
+    if #[cfg(target_os = "emscripten")] {
+        #[path = "emcc.rs"]
+        mod imp;
+    } else if #[cfg(target_arch = "wasm32")] {
+        #[path = "dummy.rs"]
+        mod imp;
+    } else if #[cfg(all(target_env = "msvc", target_arch = "aarch64"))] {
+        #[path = "dummy.rs"]
+        mod imp;
+    } else if #[cfg(target_env = "msvc")] {
+        #[path = "seh.rs"]
+        mod imp;
+    } else if #[cfg(all(windows, target_arch = "x86_64", target_env = "gnu"))] {
+        #[path = "seh64_gnu.rs"]
+        mod imp;
+    } else {
+        // Rust runtime's startup objects depend on these symbols, so make them public.
+        #[cfg(all(target_os="windows", target_arch = "x86", target_env="gnu"))]
+        pub use imp::eh_frame_registry::*;
+        #[path = "gcc.rs"]
+        mod imp;
+    }
+}
 
 mod dwarf;
 mod windows;
diff --git a/src/libpanic_unwind/macros.rs b/src/libpanic_unwind/macros.rs
new file mode 100644
index 00000000000..6ea79dc862b
--- /dev/null
+++ b/src/libpanic_unwind/macros.rs
@@ -0,0 +1,45 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+/// A macro for defining `#[cfg]` if-else statements.
+///
+/// This is similar to the `if/elif` C preprocessor macro by allowing definition
+/// of a cascade of `#[cfg]` cases, emitting the implementation which matches
+/// first.
+///
+/// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
+/// without having to rewrite each clause multiple times.
+macro_rules! cfg_if {
+    ($(
+        if #[cfg($($meta:meta),*)] { $($it:item)* }
+    ) else * else {
+        $($it2:item)*
+    }) => {
+        __cfg_if_items! {
+            () ;
+            $( ( ($($meta),*) ($($it)*) ), )*
+            ( () ($($it2)*) ),
+        }
+    }
+}
+
+macro_rules! __cfg_if_items {
+    (($($not:meta,)*) ; ) => {};
+    (($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
+        __cfg_if_apply! { cfg(all(not(any($($not),*)), $($m,)*)), $($it)* }
+        __cfg_if_items! { ($($not,)* $($m,)*) ; $($rest)* }
+    }
+}
+
+macro_rules! __cfg_if_apply {
+    ($m:meta, $($it:item)*) => {
+        $(#[$m] $it)*
+    }
+}