.travis.yml: reduce the load on [ppc64] GCC check-tcg

This seems to be timing out quite often and occasionally running out
of disk space. Relegate it to light duties.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200501111505.4225-5-alex.bennee@linaro.org>

.travis.yml

@@ -458,7 +458,7 @@ jobs:
           - genisoimage
       env:
         - TEST_CMD="make check check-tcg V=1"
-        - CONFIG="--disable-containers --target-list=${MAIN_SOFTMMU_TARGETS},ppc64le-linux-user"
+        - CONFIG="--disable-containers --target-list=ppc64-softmmu,ppc64le-linux-user"
 
     - name: "[s390x] GCC check-tcg"
       arch: s390x