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-baremetal

@@ -9,13 +9,12 @@ class Main(common.BuildCliFunction):
     def __init__(self):
         super().__init__(
             defaults={
-                'gcc_which':'crosstool-ng',
+                'gcc_which': 'crosstool-ng',
+                'mode': 'baremetal',
             },
             description='''\
 Build the baremetal examples with crosstool-NG.
 ''',
-            is_baremetal=True,
-            supported_archs=common.consts['crosstool_ng_supported_archs']
         )
         self._add_argument('--ccflags')
         self._add_argument('--force-rebuild')