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'].

userland/arch/x86_64/gdb_tests/integer_registers.py

@@ -0,0 +1,7 @@
+def test(self):
+    self.sendline('tbreak main')
+    self.sendline('continue')
+    self.continue_to('op1')
+    assert self.get_int('$rax') == 1
+    self.continue_to('result')
+    assert self.get_int('$rbx') == 3