Don't use cross for thumb target CI

Author: Amanieu

.travis.yml

@@ -64,7 +64,11 @@ matrix:
 
     # Ensure that we successfully build without libstd
     - name: "thumbv6m-none-eabi"
-      env: TARGET=thumbv6m-none-eabi CROSS=1 NO_STD=1
+      env: TARGET=thumbv6m-none-eabi NO_STD=1
+      script:
+        # cross doesn't seem to work with thumb targets...
+        - rustup target install $TARGET
+        - sh ci/run.sh
 
 install: travis_retry rustup target add "${TARGET}"
 script: sh ci/run.sh