BUILD: travis-ci: enable regular s390x builds

Previous patch didn't only disable removal of the reg-test but
disabled s390x entirely. Now we enable it like other platforms.
This is an attempt at fixing build issue #504.

.travis.yml

@@ -47,6 +47,11 @@ matrix:
     if: type != cron
     compiler: clang
     env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1d
+  - os: linux
+    arch: s390x
+    if: type != cron
+    compiler: gcc
+    env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1d
   - os: linux
     if: type == cron
     compiler: clang