Use Linker Exectuable in Link

Prepping for being able to choose a different linker exectuable. In
the future when custom linkers are supported and platform default
linking is supported the linker itself will probably want to build the
whole command rather than just providing the exectuable stirng to be
used. This works for now though.

src/compile.rs

@@ -125,7 +125,7 @@ impl Compilation {
         module.write_to_file(temp_file.path())?;
 
         // Shell out to Clang to link the final assembly
-        let output = Command::new("clang")
+        let output = Command::new(linker.cmd.executable())
             .arg(temp_file.path())
             .arg(format!("--target={}", target.triple()))
             .arg("-o")

src/compile/linker.rs

@@ -53,6 +53,17 @@ impl LinkerCommand {
     pub fn default_asset_ty(&self) -> LinkerAssetType {
         LinkerAssetType::LlvmBc
     }
+
+    /// Get the executable this command should call.
+    ///
+    pub fn executable(&self) -> &str {
+        // FIXME: instead of exposing a &str. We should
+        //        make the linker buidl the command
+        //        rather than the compiler.
+        match *self {
+            LinkerCommand::Clang => "clang"
+        }
+    }
 }
 
 impl Default for LinkerCommand {