Add a dedicated POWER9 build to the Travis CI (#2774)

* Add dedicated POWER9 build (using new syntax to ensure it runs as a P9-only containerized job rather than a VM that
might end up on P8 hardware half of the time)
* Bump gcc version for POWER9 build

.travis.yml

@@ -75,6 +75,23 @@ matrix:
         - TARGET_BOX=LINUX32
         - BTYPE="BINARY=32"
 
+    - os: linux
+      arch: ppc64le
+      dist: bionic
+      compiler: gcc
+      before_script:
+        - sudo add-apt-repository 'ppa:ubuntu-toolchain-r/test' -y
+        - sudo apt-get update
+        - sudo apt-get install gcc-9 gfortran-9 -y
+      script:
+        - make QUIET_MAKE=1  BINARY=64 USE_OPENMP=1 CC=gcc-9 FC=gfortran-9
+        - make -C test $COMMON_FLAGS $BTYPE
+        - make -C ctest $COMMON_FLAGS $BTYPE
+        - make -C utest $COMMON_FLAGS $BTYPE 
+      env:
+        # for matrix annotation only
+        - TARGET_BOX=PPC64LE_LINUX_P9
+
     - os: linux
       compiler: gcc
       addons: