test-gdb: can now run in either userland or baremetal modes

Selection with --mode userland (default because has x86_64) or --mode baremetal.

This is the first userland tool where this choice is done on the command line,
which led to a refactor of supported_archs and is_baremetal and is_userland
into a single self.env['mode'].

build-userland

@@ -15,7 +15,11 @@ class Main(common.BuildCliFunction):
             kwargs['description'] = '''\
 Build our compiled userland examples.
 '''
-        super().__init__(*args, is_userland=True, **kwargs)
+        if not 'defaults' in kwargs:
+            kwargs['defaults'] = {}
+        if not 'mode' in kwargs['defaults']:
+            kwargs['defaults']['mode'] = 'userland'
+        super().__init__(*args, **kwargs)
         self._add_argument('--ccflags')
         self._add_argument('--force-rebuild')
         self._add_argument('--optimization-level')