pipeline: Run proper cross compile host

The build-using-self script was always using macos arm64 as a cross
compile host.

Modify the build-using-self's cross compile host to use x86_64 when on
ARM, arm64 when on x86_64, and raise an exception when an any other
architecture.

Author: bkhouri

Diff for: Utilities/build-using-self

@@ -47,6 +47,11 @@ logging.basicConfig(
 REPO_ROOT_PATH = pathlib.Path(__file__).parent.parent.resolve()
 
 
+class UnsupportedArchitecture(Exception):
+
+    pass
+
+
 def get_arguments() -> argparse.Namespace:
     parser = argparse.ArgumentParser(
         formatter_class=argparse.ArgumentDefaultsHelpFormatter
@@ -89,7 +94,7 @@ def get_swiftpm_bin_dir(config: Configuration) -> pathlib.Path:
 
 
 def is_on_darwin() -> bool:
-    return platform.uname().system == "Darwin"
+    return platform.system() == "Darwin"
 
 
 def set_environment(*, swiftpm_bin_dir: pathlib.Path,) -> None:
@@ -107,22 +112,30 @@ def set_environment(*, swiftpm_bin_dir: pathlib.Path,) -> None:
 
 
 def run_bootstrap(swiftpm_bin_dir: pathlib.Path) -> None:
-    if is_on_darwin():
-        logging.info("Current working directory is %s", pathlib.Path.cwd())
-        logging.info("Bootstrapping with the XCBuild codepath...")
-        call(
-            [
-                REPO_ROOT_PATH / "Utilities" / "bootstrap",
-                "build",
-                "--release",
-                "--verbose",
-                "--cross-compile-hosts",
-                "macosx-arm64",
-                "--skip-cmake-bootstrap",
-                "--swift-build-path",
-                (swiftpm_bin_dir / "swift-build").resolve(),
-            ],
-        )
+    logging.info("Current working directory is %s", pathlib.Path.cwd())
+    logging.info("Bootstrapping with the XCBuild codepath...")
+    cross_compile_arch: str
+    arch = platform.machine()
+    if arch == "arm64":
+        cross_compile_arch = "x86_64"
+    elif arch == "x86_64":
+        cross_compile_arch = "arm64"
+    else:
+        raise UnsupportedArchitecture(f"Architecture {arch} is not supported.")
+
+    call(
+        [
+            REPO_ROOT_PATH / "Utilities" / "bootstrap",
+            "build",
+            "--release",
+            "--verbose",
+            "--cross-compile-hosts",
+            f"macosx-{cross_compile_arch}",
+            "--skip-cmake-bootstrap",
+            "--swift-build-path",
+            (swiftpm_bin_dir / "swift-build").resolve(),
+        ],
+    )
 
 
 def main() -> None:
@@ -158,7 +171,8 @@ def main() -> None:
             ),
         )
 
-    run_bootstrap(swiftpm_bin_dir=swiftpm_bin_dir)
+    if is_on_darwin():
+        run_bootstrap(swiftpm_bin_dir=swiftpm_bin_dir)
 
 
 if __name__ == "__main__":