Add support for clang-built kernels. This is completely automatic, we
check if the kernel was built with clang by looking for
CONFIG_CC_IS_CLANG in config.
This has almost no effect on non-clang built kernels with one exception:
we now do compliler checks _after_ we download kernel sources which is a
waste of resources in case when compilers don't match.
Signed-off-by: Artem Savkov <asavkov@redhat.com>
