.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: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6192)

.travis.yml

@@ -31,6 +31,10 @@ env:
 
 matrix:
     include:
+        - os: linux-ppc64le
+          sudo: false
+          compiler: clang
+          env: CONFIG_OPTS="--strict-warnings -D__NO_STRING_INLINES"
         - os: linux
           addons:
               apt:
@@ -52,6 +56,12 @@ matrix:
                       - gcc-mingw-w64
           compiler: i686-w64-mingw32-gcc
           env: CONFIG_OPTS="no-stdio" BUILDONLY="yes"
+        # Uncomment if there is reason to believe that PPC-specific problem
+        # can be diagnosed with this possibly >30 mins sanitizer build...
+        #- os: linux-ppc64le
+        #  sudo: false
+        #  compiler: gcc
+        #  env: EXTENDED_TEST="yes" CONFIG_OPTS="no-asm enable-asan enable-ubsan no-shared -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -D__NO_STRING_INLINES"
         - os: linux
           addons:
               apt: