default-configs: Add config for aarch64-linux-user

Add a config for aarch64-linux-user, thereby enabling it as
a valid target.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAlexander Graf <agraf@suse.de>
Reviewed-by: NRichard Henderson <rth@twiddle.net>

default-configs/aarch64-linux-user.mak

+# Default configuration for aarch64-linux-user
+
+CONFIG_GDBSTUB_XML=y