Building perf with BUILD_BPF_SKEL=1 by default

[Arnaldo Carvalho de Melo]

Hi Song,

	So I'm changing all my containers to build with BUILD_BPF_SKEL=1
to then make this the default, so far older containers fail either
because the clang available is too old, so I've added a NO_BUILD_BPF_SKEL=1
env var to disable that in those containers and then there is this other
case where clang is recent enough but:

    util/bpf_skel/bperf_leader.bpf.c:13:20: error: use of undeclared identifier 'BPF_F_PRESERVE_ELEMS'
            __uint(map_flags, BPF_F_PRESERVE_ELEMS);

Because the system's /usr/include/linux/bpf.h doesn't have that
BPF_F_PRESERVE_ELEMS enum entry.

These are enums to make them available via BTF, but then I can't use
the:

#ifdef BPF_F_PRESERVE_ELEMS
#define BPF_F_PRESERVE_ELEMS (1U << 11)
#endif

approach.

But then we _have_ it in the tools/include/uapi/linux/bpf.h we ship:

$ grep BPF_F_PRESERVE_ELEMS tools/include/uapi/linux/bpf.h
	BPF_F_PRESERVE_ELEMS	= (1U << 11),
$

so we need to switch to using it somehow, this way we can build in more
systems and make bperf and other BPF enabled features.

>From a quick look I couldn't find where to add
$(sourcedir)/tools/include/uapi/ to the include path used to build
util/bpf_skel/bperf_leader.bpf.c, should be easy, can you take a look?

Thanks,

- Arnaldo