Make userland / assembly getting started more uniform / visible

Forward --gcc-which to ./run --tmux.

Use gdb-multiarch for --gcc-which host.

run-gdb

@@ -153,7 +153,7 @@ See: https://github.com/cirosantilli/linux-kernel-module-cheat#gdb-builtin-cpu-s
         else:
             image = self.env['vmlinux']
         cmd = (
-            [self.get_toolchain_tool('gdb'), LF] +
+            [self.env['gdb_path'], LF] +
             before
         )
         if linux_full_system: