selftests/bpf: do not ignore clang failures

selftests/bpf: do not ignore clang failures

[ Upstream commit 9cae4ace80ef39005da106fbb89c952b27d7b89e ]

When compiling an eBPF prog fails, make still returns 0, because
failing clang command's output is piped to llc and therefore its
exit status is ignored.

When clang fails, pipe the string "clang failed" to llc. This will make
llc fail with an informative error message. This solution was chosen
over using pipefail, having separate targets or getting rid of llc
invocation due to its simplicity.

In addition, pull Kbuild.include in order to get .DELETE_ON_ERROR target,
which would cause partial .o files to be removed.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>