tools lib bpf: Use FEATURE_USER to allow building in the same dir as perf

When building tools/lib/bpf as part of the tools/perf/ build process,
which will happend when we introduce a patch wiring that up, we end up
stomping on the feature detection caching mechanism, that uses a file in
the output directory (O=) that is shared by libbpf and perf to check if
something changed from one build to another that requires redoing the
feature detection process.

By using the recently introduced FEATURE_USER tools/build/ knob, we can
avoid that:

Before, every make invokation would run the feature detection:

  $ make O=/tmp/build/perf -C tools/perf
  make: Entering directory '/home/git/linux/tools/perf'
  Auto-detecting system features:
  ...                         dwarf: [ on  ]
  ...                         glibc: [ on  ]
  <SNIP>
  ...                     get_cpuid: [ on  ]
  ...                           bpf: [ on  ]

    GEN      perf-archive
    GEN      perf-with-kcore

  Auto-detecting system features:
  ...                        libelf: [ on  ]
  ...                           bpf: [ on  ]
  <SNIP>

After:

  $ make O=/tmp/build/perf -C tools/perf
  make: Entering directory '/home/git/linux/tools/perf'
    BUILD:   Doing 'make -j4' parallel build
  make: Leaving directory '/home/git/linux/tools/perf'
  $

Because we now have two different feature detection state files:

  $ ls -la /tmp/build/perf/FEATURE-DUMP*
  -rw-rw-r--. 1 acme acme 338 Sep 21 17:25 /tmp/build/perf/FEATURE-DUMP
  -rw-rw-r--. 1 acme acme  33 Sep 21 17:25 /tmp/build/perf/FEATURE-DUMP.libbpf
  $

Acked-by: Jiri Olsa <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Wang Nan <[email protected]>
Cc: [email protected]
Fixes: 1b76c13e4b36 ("bpf tools: Introduce 'bpf' library and add bpf feature check")
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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