perf build: Allow customization of clang options for BPF target

perf build: Allow customization of clang options for BPF target

This also puts an unconditional -Werror under control of WERROR. The
clang includes added during the build can lead to a warning that may be
turned into an error. In addition, hardened clang produces a warning
about lack of support for -fstack-protector* options for the BPF target:

  clang -g -O2 -target bpf -Wall -Werror -Ilinux/tools/perf/util/bpf_skel/.tmp/.. \
    -I -idirafter /usr/lib/llvm/16/bin/../../../../lib/clang/16/include -idirafter /usr/local/include \
    -idirafter /usr/include  -Ilinux/tools/include/uapi -c util/bpf_skel/bperf_follower.bpf.c \
    -o linux/tools/perf/util/bpf_skel/.tmp/bperf_follower.bpf.o && llvm-strip -g linux/tools/perf/util/bpf_skel/.tmp/bperf_follower.bpf.o
  clang-16: error: /usr/lib/llvm/16/bin/../../../../lib/clang/16/include: 'linker' input unused [-Werror,-Wunused-command-line-argument]
  clang-16: error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]
  make[1]: *** [Makefile.perf:1082: linux/tools/perf/util/bpf_skel/.tmp/bpf_prog_profiler.bpf.o] Error 1

Signed-off-by: Guilherme Amadio <amadio@gentoo.org>
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>
Link: https://lore.kernel.org/lkml/ZOZQ2LDA+3Wg8x2T@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>