Support the x86_64-linux-android build target. (#127)

* Initial support for x86_64-linux-android targets.

* Improve target selection for clang link phase.

* Only add clang target if it differs from the host.

Author: jsclary

xbuild/src/cargo/mod.rs

@@ -295,7 +295,9 @@ impl CargoBuild {
             target_sdk_version
         );
         self.use_ld("lld");
-        self.add_link_arg("--target=aarch64-linux-android");
+        if let Some(triple) = self.triple {
+            self.add_link_arg(&format!("--target={}", triple));
+        }
         self.add_link_arg(&format!("-B{}", sdk_lib_dir.display()));
         self.add_link_arg(&format!("-L{}", sdk_lib_dir.display()));
         self.add_link_arg(&format!("-L{}", lib_dir.display()));