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-crosstool-ng

@@ -8,10 +8,12 @@ from shell_helpers import LF
 class Main(common.BuildCliFunction):
     def __init__(self):
         super().__init__(
+            defaults={
+                'mode': 'baremetal',
+            },
             description='''\
 Build crosstool-NG with Newlib for bare metal compilation
 ''',
-            supported_archs=common.consts['crosstool_ng_supported_archs']
         )
 
     def build(self):