Add ability to specify target for Microkit tool

Add the `--tool-target-triple` option to specify which target
to build the tool for. Unfortunately Rust does not allow you to
cross-compile from Linux to macOS, however this will allow x64 macOS
hosts to cross-compile to ARM64 macOS. In theory the reverse would
work as well but I have not tried it.

Signed-off-by: Ivan Velickovic <[email protected]>

build_sdk.py

@@ -152,32 +152,34 @@ def tar_filter(tarinfo: TarInfo) -> TarInfo:
     return tarinfo
 
 
-def test_tool() -> None:
-    r = system(
-        f"{executable} -m unittest discover -s tool -v"
-    )
-    assert r == 0
-
-
-def build_tool(tool_target: Path) -> None:
-    pyoxidizer = ENV_BIN_DIR / "pyoxidizer"
-    if not pyoxidizer.exists():
-        raise Exception("pyoxidizer does not appear to be installed in your Python environment")
-
+def get_tool_target_triple() -> str:
     host_system = host_platform.system()
     if host_system == "Linux":
-        target_triple = "x86_64-unknown-linux-musl"
+        return "x86_64-unknown-linux-musl"
     elif host_system == "Darwin":
         host_arch = host_platform.machine()
         if host_arch == "x86_64":
-            target_triple = "x86_64-apple-darwin"
+            return "x86_64-apple-darwin"
         elif host_arch == "arm64":
-            target_triple = "aarch64-apple-darwin"
+            return "aarch64-apple-darwin"
         else:
             raise Exception(f"Unexpected Darwin architecture: {host_arch}")
     else:
         raise Exception(f"The platform \"{host_system}\" is not supported")
 
+
+def test_tool() -> None:
+    r = system(
+        f"{executable} -m unittest discover -s tool -v"
+    )
+    assert r == 0
+
+
+def build_tool(tool_target: Path, target_triple: str) -> None:
+    pyoxidizer = ENV_BIN_DIR / "pyoxidizer"
+    if not pyoxidizer.exists():
+        raise Exception("pyoxidizer does not appear to be installed in your Python environment")
+
     r = system(
         f"{pyoxidizer} build --release --path tool --target-triple {target_triple}"
     )
@@ -357,6 +359,7 @@ def build_lib_component(
 def main() -> None:
     parser = ArgumentParser()
     parser.add_argument("--sel4", type=Path, required=True)
+    parser.add_argument("--tool-target-triple", default=get_tool_target_triple(), help="Compile the Microkit tool for this target triple")
     args = parser.parse_args()
     sel4_dir = args.sel4.expanduser()
     if not sel4_dir.exists():
@@ -402,7 +405,7 @@ def main() -> None:
 
     if not tool_target.exists():
         test_tool()
-        build_tool(tool_target)
+        build_tool(tool_target, args.tool_target_triple)
 
     build_doc(root_dir)