travis.yml: Rework the aarch64 jobs

With the new QEMU from Ubuntu Focal, we can now run some more tests
with TCG. Also switch the second build job to native arm64, so we
can use Clang to compile these tests to get some additional compiler
test coverage.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20201001072234.143703-8-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>