use defconfig for ct-ng, minimizing the config (cross-rs#1468)

This PR switches to using `defconfig` for ct-ng, minimizing the config.

The `docker/crosstool-config/loongarch64-unknown-linux-gnu.config` is
generated by:

```sh
GCC_VERSION=13.2.0 GLIBC_VERSION=2.36 LINUX_VERSION=5.19.16 cargo xtask configure-crosstool
```

and `docker/crosstool-config/arm-unknown-linux-gnueabihf.config`:

```sh
GCC_VERSION=8.5.0 GLIBC_VERSION=2.31 LINUX_VERSION=4.19.287 cargo xtask configure-crosstool
```

Fixes cross-rs#1335

.changes/1468.json

@@ -0,0 +1,5 @@
+{
+    "type": "changed",
+    "description": "use defconfig for ct-ng, minimizing the config",
+    "issues": [1335]
+}

docker/crosstool-ng.sh

@@ -71,6 +71,7 @@ main() {
     pushd "${buildir}"
     cp /"${config}" .config
     chown "${username}":"${username}" .config
+    su "${username}" -c "${crosstooldir}/bin/ct-ng olddefconfig"
 
     # the download steps can stall indefinitely, so we want to set a timeout to
     # ensure it always completes. we therefore attempt to  download until