Auto merge of #47018 - malbarbo:armv4t, r=alexcrichton

Add armv4t-unknown-linux-gnueabi target

armv4t was left out of #37615 (armv5te addition) to be included in a [future PR](#37615 (comment)).  So this PR adds armv4t target.

armv4t target is useful because the [armel](https://wiki.debian.org/ArmEabiPort) port of Debian targets armv4t

Author: bors

src/libcompiler_builtins

@@ -0,0 +1,36 @@
+// Copyright 2016 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 LinkerFlavor;
+use target::{Target, TargetOptions, TargetResult};
+
+pub fn target() -> TargetResult {
+    let base = super::linux_base::opts();
+    Ok(Target {
+        llvm_target: "armv4t-unknown-linux-gnueabi".to_string(),
+        target_endian: "little".to_string(),
+        target_pointer_width: "32".to_string(),
+        target_c_int_width: "32".to_string(),
+        data_layout: "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64".to_string(),
+        arch: "arm".to_string(),
+        target_os: "linux".to_string(),
+        target_env: "gnu".to_string(),
+        target_vendor: "unknown".to_string(),
+        linker_flavor: LinkerFlavor::Gcc,
+
+        options: TargetOptions {
+            features: "+soft-float,+strict-align".to_string(),
+            // Atomic operations provided by compiler-builtins
+            max_atomic_width: Some(32),
+            abi_blacklist: super::arm_base::abi_blacklist(),
+            .. base
+        }
+    })
+}

src/librustc_back/target/armv4t_unknown_linux_gnueabi.rs

@@ -27,11 +27,7 @@ pub fn target() -> TargetResult {
 
         options: TargetOptions {
             features: "+soft-float,+strict-align".to_string(),
-
-            // Atomic operations provided when linked with libgcc.
-            // FIXME: If the following PR is merged, the atomic operations would be
-            // provided by compiler-builtins instead with no change of behavior:
-            // https://github.com/rust-lang-nursery/compiler-builtins/pull/115/files
+            // Atomic operations provided by compiler-builtins
             max_atomic_width: Some(32),
             abi_blacklist: super::arm_base::abi_blacklist(),
             .. base

src/librustc_back/target/armv5te_unknown_linux_gnueabi.rs

@@ -151,6 +151,7 @@ supported_targets! {
     ("arm-unknown-linux-gnueabihf", arm_unknown_linux_gnueabihf),
     ("arm-unknown-linux-musleabi", arm_unknown_linux_musleabi),
     ("arm-unknown-linux-musleabihf", arm_unknown_linux_musleabihf),
+    ("armv4t-unknown-linux-gnueabi", armv4t_unknown_linux_gnueabi),
     ("armv5te-unknown-linux-gnueabi", armv5te_unknown_linux_gnueabi),
     ("armv7-unknown-linux-gnueabihf", armv7_unknown_linux_gnueabihf),
     ("armv7-unknown-linux-musleabihf", armv7_unknown_linux_musleabihf),