scripts/build: add clang targets

With this, one can do a CRUI build via QEMU+Docker using clang
as a compiler, for example:

	make -C scripts/build alpine-clang
	make -C scripts/build ppc64le-clang

For alpine, default clang version is used. For others, a version has
to be specified explicitly, as there is no package called "clang" in
Ubuntu Trusty (which is used as a base for most arches). Since
clang-3.8 appears to be available from Trusty, and this is the current
stable version of clang, let's use it.

travis-ci: success for QEMU+Dockerfile improvements + clang build
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>