samples/bpf: Clean up samples/bpf build failes

Remove xdp_samples_user.o rule redefinition which generates Makefile
warning and instead override TPROGS_CFLAGS. This seems to work fine when
building inside selftests/bpf.

That was one big head-scratcher before I found that generic
Makefile.target hid this surprising specialization for for xdp_samples_user.o.

Main change is to use actual locally installed libbpf headers.

Also drop printk macro re-definition (not even used!).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211201232824.3166325-8-andrii@kernel.org

samples: bpf: Convert xdp_monitor to XDP samples helper

Use the libbpf skeleton facility and other utilities provided by XDP
samples helper.

A lot of the code in xdp_monitor and xdp_redirect_cpu has been moved to
the xdp_sample_user.o helper, so we remove the duplicate functions here
that are no longer needed.

Thanks to BPF skeleton, we no longer depend on order of tracepoints to
uninstall them on startup. Instead, the sample mask is used to install
the needed tracepoints.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210821002010.845777-15-memxor@gmail.com

samples/bpf: Add makefile.target for separate CC target build

The Makefile.target is added only and will be used in
sample/bpf/Makefile later in order to switch cross-compiling to CC
from HOSTCC environment.

The HOSTCC is supposed to build binaries and tools running on the host
afterwards, in order to simplify build or so, like "fixdep" or else.
In case of cross compiling "fixdep" is executed on host when the rest
samples should run on target arch. In order to build binaries for
target arch with CC and tools running on host with HOSTCC, lets add
Makefile.target for simplicity, having definition and routines similar
to ones, used in script/Makefile.host. This allows later add
cross-compilation to samples/bpf with minimum changes.

The tprog stands for target programs built with CC.

Makefile.target contains only stuff needed for samples/bpf, potentially
can be reused later and now needed only for unblocking tricky
samples/bpf cross compilation.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191011002808.28206-8-ivan.khoronzhuk@linaro.org