xcode: Autodetect host machine's CPU architecture

Currently, the backend has "x86_64" hardcoded as the architecture,
which breaks cross compiling and compiling normally on arm64. Fix
this by setting it to the host machine's CPU architecture instead.

mesonbuild/backend/xcodebackend.py

@@ -195,6 +195,9 @@ class XCodeBackend(backends.Backend):
         self.regen_dependency_id = self.gen_id()
         self.top_level_dict = PbxDict()
         self.generator_outputs = {}
+        self.arch = self.build.environment.machines.host.cpu
+        if self.arch == 'aarch64':
+            self.arch = 'arm64'
         # In Xcode files are not accessed via their file names, but rather every one of them
         # gets an unique id. More precisely they get one unique id per target they are used
         # in. If you generate only one id per file and use them, compilation will work but the
@@ -233,11 +236,10 @@ class XCodeBackend(backends.Backend):
         project = self.build.project_name
         buildtype = self.buildtype
         tname = target.get_id()
-        arch = 'x86_64'
         if isinstance(source, mesonlib.File):
             source = source.fname
         stem = os.path.splitext(os.path.basename(source))[0]
-        obj_path = f'{project}.build/{buildtype}/{tname}.build/Objects-normal/{arch}/{stem}.o'
+        obj_path = f'{project}.build/{buildtype}/{tname}.build/Objects-normal/{self.arch}/{stem}.o'
         return obj_path
 
     def determine_swift_dep_dirs(self, target: build.BuildTarget) -> T.List[str]:
@@ -1391,7 +1393,7 @@ class XCodeBackend(backends.Backend):
             bt_dict.add_item('isa', 'XCBuildConfiguration')
             settings_dict = PbxDict()
             bt_dict.add_item('buildSettings', settings_dict)
-            settings_dict.add_item('ARCHS', '"$(NATIVE_ARCH_ACTUAL)"')
+            settings_dict.add_item('ARCHS', f'"{self.arch}"')
             settings_dict.add_item('ONLY_ACTIVE_ARCH', 'YES')
             settings_dict.add_item('SWIFT_VERSION', '5.0')
             settings_dict.add_item('SDKROOT', '"macosx"')
@@ -1435,7 +1437,7 @@ class XCodeBackend(backends.Backend):
             bt_dict.add_item('isa', 'XCBuildConfiguration')
             settings_dict = PbxDict()
             bt_dict.add_item('buildSettings', settings_dict)
-            settings_dict.add_item('ARCHS', '"$(NATIVE_ARCH_ACTUAL)"')
+            settings_dict.add_item('ARCHS', f'"{self.arch}"')
             settings_dict.add_item('ONLY_ACTIVE_ARCH', 'YES')
             settings_dict.add_item('SDKROOT', '"macosx"')
             settings_dict.add_item('SYMROOT', '"%s/build"' % self.environment.get_build_dir())