rustbuild: per target musl-root

config.toml now accepts a target.$TARGET.musl-root key that lets you
override the "build" musl-root value, which is set via the --musl-root
flag or via the build.musl-root key.

With this change, it's now possible to compile std for several musl
targets at once. Here's are the sample commands to do such thing:

```
$ configure \
    --enable-rustbuild \
    --target=x86_64-unknown-linux-musl,arm-unknown-linux-musleabi \
    --musl-root=/musl/x86_64-unknown-linux-musl/

$ edit config.toml && tail config.toml
[target.arm-unknown-linux-musleabi]
musl-root = "/x-tools/arm-unknown-linux-musleabi/arm-unknown-linux-musleabi/sysroot/usr"

$ make
```

1 files changed, 3 insertions, 2 deletions

diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index 302ac68460c..e14317b23b4 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -59,8 +59,9 @@ pub fn std<'a>(build: &'a Build, target: &str, compiler: &Compiler<'a>) {
             cargo.env("JEMALLOC_OVERRIDE", jemalloc);
         }
     }
-    if let Some(ref p) = build.config.musl_root {
-        if target.contains("musl") {
+    if target.contains("musl") {
+        if let Some(p) = build.config.target_config[target].musl_root.as_ref()
+            .or(build.config.musl_root.as_ref()) {
             cargo.env("MUSL_ROOT", p);
         }
     }