The later handler if conditionally compiled only for Linux but we
forgot to ensure we don't advertise it lest we confuse our BSD
brethren.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Fixes: 51c623b0 ("gdbstub: add support to Xfer:auxv:read: packet")
Reported-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Tested-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20230403134920.2132362-4-alex.bennee@linaro.org>