Scope build files for non-native targets

This ensures we don't end up using the wrong data (e.g. object file
caches) when compiling for different targets.

Changelog: changed

compiler/src/config.rs

@@ -58,10 +58,14 @@ pub(crate) struct BuildDirectories {
 
 impl BuildDirectories {
     pub(crate) fn new(config: &Config) -> BuildDirectories {
-        let build = config
-            .opt
-            .directory_name()
-            .map_or(config.build.clone(), |p| config.build.join(p));
+        let root = if config.target.is_native() {
+            config.build.clone()
+        } else {
+            config.build.join(config.target.to_string())
+        };
+
+        let build =
+            config.opt.directory_name().map(|p| root.join(p)).unwrap_or(root);
 
         let objects = build.join("objects");
         let llvm_ir = build.join("llvm");

docs/source/getting-started/cli.md

@@ -54,4 +54,10 @@ You can specify an alternative output path using the `-o` option:
 inko build -o /tmp/hello hello.inko
 ```
 
+When compiling for the host/native target, build output is placed in `./build`
+directly, but when building for a different architecture the output is scoped to
+a directory named after that architecture. For example, when compiling for
+arm64-linux-gnu on an amd64-linux-gnu host, build files are placed in
+`./build/arm64-linux-gnu`.
+
 For more information, run `inko --help`.