Merge bitcoin#696: Run a Travis test on s390x (big endian)

3929536 Test travis s390x (big endian) (Pieter Wuille)

Pull request description:

ACKs for top commit:
  real-or-random:
    ACK 3929536 Travis works and says it's big endian

Tree-SHA512: 939b98fe369e575e8bf56899a28cb5aafdb9ccfaaee3cb611027e053edc8220d2787c34359cd01508899b8b7e105c89853a4ab44c382252538c797d00c09345b

.travis.yml

@@ -83,6 +83,10 @@ matrix:
             - valgrind
             - libtool-bin
             - libc6-dbg:i386
+    # S390x build (big endian system)
+    - compiler: gcc
+      env: HOST=s390x-unknown-linux-gnu ECDH=yes RECOVERY=yes EXPERIMENTAL=yes CTIMETEST=
+      arch: s390x
 
 # We use this to install macOS dependencies instead of the built in `homebrew` plugin,
 # because in xcode earlier than 11 they have a bug requiring updating the system which overall takes ~8 minutes.