Auto merge of #32034 - alexcrichton:old-x86-msvc, r=aturon

rustc: Add an i586-pc-windows-msvc target

Similarly to #31629 where an i586-unknown-linux-gnu target was added, there is
sometimes a desire to compile for x86 Windows as well where SSE2 is disabled.
This commit mirrors the i586-unknown-linux-gnu target and simply adds a variant
for Windows as well.

This is motivated by a recent [Gecko bug][ff] where crashes were seen on 32-bit
Windows due to users having CPUs that don't support SSE2 instructions. It was
requested that we could have non-SSE2 builds of the standard library available
so they could continue to use vanilla releases and nightlies.

[ff]: https://bugzilla.mozilla.org/show_bug.cgi?id=1253202

src/librustc_back/target/i586_pc_windows_msvc.rs

@@ -0,0 +1,18 @@
+// Copyright 2015 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.
+
+use target::Target;
+
+pub fn target() -> Target {
+    let mut base = super::i686_pc_windows_msvc::target();
+    base.options.cpu = "pentium".to_string();
+    base.llvm_target = "i586-pc-windows-msvc".to_string();
+    return base
+}

src/librustc_back/target/i586_unknown_linux_gnu.rs

@@ -11,18 +11,8 @@
 use target::Target;
 
 pub fn target() -> Target {
-    let mut base = super::linux_base::opts();
-    base.cpu = "pentium".to_string();
-    base.pre_link_args.push("-m32".to_string());
-
-    Target {
-        llvm_target: "i586-unknown-linux-gnu".to_string(),
-        target_endian: "little".to_string(),
-        target_pointer_width: "32".to_string(),
-        arch: "x86".to_string(),
-        target_os: "linux".to_string(),
-        target_env: "gnu".to_string(),
-        target_vendor: "unknown".to_string(),
-        options: base,
-    }
+    let mut base = super::i686_unknown_linux_gnu::target();
+    base.options.cpu = "pentium".to_string();
+    base.llvm_target = "i586-unknown-linux-gnu".to_string();
+    return base
 }

src/librustc_back/target/mod.rs

@@ -136,6 +136,7 @@ supported_targets! {
 
     ("x86_64-pc-windows-msvc", x86_64_pc_windows_msvc),
     ("i686-pc-windows-msvc", i686_pc_windows_msvc),
+    ("i586-pc-windows-msvc", i586_pc_windows_msvc),
 
     ("le32-unknown-nacl", le32_unknown_nacl),
     ("asmjs-unknown-emscripten", asmjs_unknown_emscripten)