.travis.yml: add gcc sanitizer build

As it seems easy to break the ThreadSanitizer build we should defend it to
ensure that fixes get applied when it breaks. We use the Ubuntu GCC PPA
to get the latest GCC goodness.

As we need to use the -fuse-ld=gold work around we have to disable the
linux-user targets as these trip up the linker.

The make check run is also disabled for Travis but this can be
re-enabled once the check targets have been fixed.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20160930213106.20186-13-alex.bennee@linaro.org>
