travis: Build on ARM64, IBM Power and IBM Z architectures

IBM Z is big-endian, IBM Power runs in little-endian mode.

Botan requires a fix for issues with GCC and amalgamation enabled (target
pragma ‘*’ is invalid) on ARM64 and IBM Power, while wolfSSL can't be
compiled successfully on IBM Z without an additional patch.

libunwind is not available for x390x, but since we explicitly disable
such backtraces it's not necessary anyway.

.travis.yml

@@ -50,6 +50,12 @@ matrix:
     - env: TEST=all
     - env: TEST=all
       compiler: clang
+    - env: TEST=all
+      arch: arm64
+    - env: TEST=all
+      arch: ppc64le
+    - env: TEST=all
+      arch: s390x
     - env: TEST=all
       dist: xenial
     - env: TEST=all