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

@@ -318,8 +318,8 @@ so looping over all of them would waste time.
             ]),
             'test-gdb': _Component(dependencies=[
                     'all-baremetal',
+                    'userland',
                 ],
-                supported_archs=common.consts['crosstool_ng_supported_archs'],
             ),
             'test-user-mode': _Component(dependencies=[
                 'test-user-mode-qemu',