gdb: switch "set language" to getter/setter

The `language` global variable is mostly a scratch variable used for the
setting.  The source of truth is really current_language and
language_mode (auto vs manual), which are set by the
set_language_command callback.

Switch the setting to use the add_setshow_enum_cmd overload that takes a
value getter and setter.

Change-Id: Ief5b2f93fd7337eed7ec96023639ae3dfe62250b
Reviewed-By: Tom Tromey <tom@tromey.com>