rustbuild: Fix bug preventing per-target musl-root

In #36292, support was added to target musl libc for ARM targets using
rustbuild. Specifically, that change allowed the addition of per-target
"musl-root" options in the rustbuild config.toml so that multiple
targets depending on musl could be built. However, that implementation
contained a couple of omissions: the musl-root option was added to the
config, but was never added to the TOML parsing, and therefore was not
actually being loaded from config.toml. This commit rectifies that and
allows successful building of musl-based ARM targets.

src/bootstrap/compile.rs

@@ -90,16 +90,16 @@ pub fn std_link(build: &Build,
     add_to_sysroot(&out_dir, &libdir);
 
     if target.contains("musl") && !target.contains("mips") {
-        copy_musl_third_party_objects(build, &libdir);
+        copy_musl_third_party_objects(build, target, &libdir);
     }
 }
 
 /// Copies the crt(1,i,n).o startup objects
 ///
 /// Only required for musl targets that statically link to libc
-fn copy_musl_third_party_objects(build: &Build, into: &Path) {
+fn copy_musl_third_party_objects(build: &Build, target: &str, into: &Path) {
     for &obj in &["crt1.o", "crti.o", "crtn.o"] {
-        copy(&build.config.musl_root.as_ref().unwrap().join("lib").join(obj), &into.join(obj));
+        copy(&build.musl_root(target).unwrap().join("lib").join(obj), &into.join(obj));
     }
 }
 

src/bootstrap/config.rs

@@ -158,6 +158,7 @@ struct TomlTarget {
     cc: Option<String>,
     cxx: Option<String>,
     android_ndk: Option<String>,
+    musl_root: Option<String>,
 }
 
 impl Config {
@@ -268,6 +269,7 @@ impl Config {
                 }
                 target.cxx = cfg.cxx.clone().map(PathBuf::from);
                 target.cc = cfg.cc.clone().map(PathBuf::from);
+                target.musl_root = cfg.musl_root.clone().map(PathBuf::from);
 
                 config.target_config.insert(triple.clone(), target);
             }