Add arm64 in test matrix on TravisCI.

Change-Id: I5d2b729699cfd8e80c3df17db4a9d2edcbf64454

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from #10046)

(cherry picked from commit 0399aba)

.travis.yml

@@ -32,6 +32,10 @@ env:
 
 matrix:
     include:
+        - os: linux
+          arch: arm64
+          compiler: gcc
+          env: CONFIG_OPTS="--strict-warnings"
         - os: linux-ppc64le
           sudo: false
           compiler: clang