configure: add ppc target back to container tests

Commit 2ff8c9a298 removed support for 32-bit PPC hosts from the build
system. Unfortunately, the patch also removed the 32-bit PPC target for
containerized tests, which leads to an error when trying to run tests,
e.g., with "make check-tcg":
"make[1]: *** No rule to make target 'docker-image-debian-ppc-cross',
needed by 'build-tcg-tests-ppc-linux-user'.  Stop."

This patch adds the PPC target back for containerized tests.

Fixes: 2ff8c9a298 ("buildsys: Remove support for 32-bit PPC hosts")
Signed-off-by: Florian Hofhammer <florian.hofhammer@fhofhammer.de>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
[Mjt: specify commit subject in Fixes tag)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

configure

@@ -1451,7 +1451,7 @@ probe_target_compiler() {
         container_image=debian-all-test-cross
         container_cross_prefix=mips64-linux-gnuabi64-
         ;;
-      ppc64|ppc64le)
+      ppc|ppc64|ppc64le)
         container_image=debian-all-test-cross
         container_cross_prefix=powerpc${target_arch#ppc}-linux-gnu-
         ;;