.travis.yml: add pair of linux-ppc64le targets.

One is clang --strict-warnings and one gcc sanitizer extended test.
Sanitizer build is quite expensive, can take >30 mins and is commented
for occasions when there is reason to believe that PPC-specific problem
can be diagnosed with sanitizer.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6192)