allow nostd test to fail, see rust-lang/rust#48734

Author: newpavlov

.travis.yml

@@ -16,7 +16,9 @@ matrix:
       rust: stable
     - env: TARGET=powerpc64-unknown-linux-gnu
       rust: stable
-    # tests if crates truly can be built without std
+  # tests if crates truly can be built without std
+  # allowed to fail due to the https://github.com/rust-lang/rust/issues/48734
+  allow_failures:
     - env: TARGET=thumbv7em-none-eabi
       rust: nightly
       script: ./build_nostd.sh