Add ARM MUSL targets.

The targets are:
- `arm-unknown-linux-musleabi`
- `arm-unknown-linux-musleabihf`
- `armv7-unknown-linux-musleabihf`

These mirror the existing `gnueabi` targets.

All of these targets produce fully static binaries, similar to the
x86 MUSL targets.

For now these targets can only be used with `--rustbuild` builds, as
https://github.com/rust-lang/compiler-rt/pull/22 only made the
necessary compiler-rt changes in the CMake configs, not the plain
GNU Make configs.

I've tested these targets GCC 5.3.0 compiled again musl-1.1.12
(downloaded from http://musl.codu.org/). An example `./configure`
invocation is:

```
./configure \
    --enable-rustbuild
    --target=arm-unknown-linux-musleabi \
    --musl-root="$MUSL_ROOT"
```

where `MUSL_ROOT` points to the `arm-linux-musleabi` prefix.
Usually that path will be of the form
`/foobar/arm-linux-musleabi/arm-linux-musleabi`.

Usually the cross-compile toolchain will live under
`/foobar/arm-linux-musleabi/bin`. That path should either by added
to your `PATH` variable, or you should add a section to your
`config.toml` as follows:

```
[target.arm-unknown-linux-musleabi]
cc = "/foobar/arm-linux-musleabi/bin/arm-linux-musleabi-gcc"
cxx = "/foobar/arm-linux-musleabi/bin/arm-linux-musleabi-g++"
```

As a prerequisite you'll also have to put a cross-compiled static
`libunwind.a` library in `$MUSL_ROOT/lib`. This is similar to [how
the x86_64 MUSL targets are built]
(https://doc.rust-lang.org/book/advanced-linking.html).

2 files changed, 2 insertions, 3 deletions

diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index 8ec9c7f0109..061192ebd13 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -92,8 +92,7 @@ pub fn std_link(build: &Build,
     }
     add_to_sysroot(&out_dir, &libdir);
 
-    if target.contains("musl") &&
-       (target.contains("x86_64") || target.contains("i686")) {
+    if target.contains("musl") && !target.contains("mips") {
         copy_third_party_objects(build, target, &libdir);
     }
 }
diff --git a/src/bootstrap/sanity.rs b/src/bootstrap/sanity.rs
index 73749246758..09f96782e71 100644
--- a/src/bootstrap/sanity.rs
+++ b/src/bootstrap/sanity.rs
@@ -109,7 +109,7 @@ pub fn check(build: &mut Build) {
         }
 
         // Make sure musl-root is valid if specified
-        if target.contains("musl") && (target.contains("x86_64") || target.contains("i686")) {
+        if target.contains("musl") && !target.contains("mips") {
             match build.config.musl_root {
                 Some(ref root) => {
                     if fs::metadata(root.join("lib/libc.a")).is_err() {