travis.yml: Add x86 build with clang 10

.gitlab-ci.yml already has a job to build the tests with clang but it's
not clear how to set it up on a personal github repo.

NB, realmode test is disabled because it fails immediately after start
if compiled with clang-10.

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200901085056.33391-11-r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>