.oniro-ci,.gitlab-ci.yml: add optional clang jobs

Existing unqualified jobs rely on defaults. Qualified jobs explicitly
configure either clang or gcc and use the appropriate suffix. Qualified
jobs inhabit two new lanes: build-gcc and build-clang, to visually break
from the already long column of "build" jobs.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>