docs: Set LIBC=gnu for s390x too
The Developer Guide instructs to install the agent from
`${ARCH}-unknown-linux-${LIBC}`, where `$LIBC` is set to `gnu` for
ppc64le (because Rust has no musl target there). The same is true for
s390x. Also set this for s390x.
Fixes:
@@ -354,10 +354,11 @@ You MUST choose one of `alpine`, `centos`, `clearlinux`, `euleros`, and `fedora`
>
> - Check the [compatibility matrix](../tools/osbuilder/README.md#platform-distro-compatibility-matrix) before creating rootfs.
-Optionally, add your custom agent binary to the rootfs with the following, `LIBC` default is `musl`, if `ARCH` is `ppc64le`, should set the `LIBC=gnu` and `ARCH=powerpc64le`:
+Optionally, add your custom agent binary to the rootfs with the following commands. The default `$LIBC` used
+is `musl`, but on ppc64le and s390x, `gnu` should be used. Also, Rust refers to ppc64le as `powerpc64le`:
```
$ export ARCH=$(uname -m)
-$ [ ${ARCH} == "ppc64le" ] && export LIBC=gnu || export LIBC=musl
+$ [ ${ARCH} == "ppc64le" ] || [ ${ARCH} == "s390x" ] && export LIBC=gnu || export LIBC=musl
$ [ ${ARCH} == "ppc64le" ] && export ARCH=powerpc64le
$ sudo install -o root -g root -m 0550 -T ../../../src/agent/target/${ARCH}-unknown-linux-${LIBC}/release/kata-agent ${ROOTFS_DIR}/sbin/init
```
