perf bpf: Do not use llvm-strip on BPF binary

llvm-strip is not really required. Remove this dependency to make it
easier to build perf with BUILD_BPF_SKEL=1.

Committer notes:

This removes the need for the 'llvm' package just to get llvm-strip.

Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>