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.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.perf

@@ -1056,10 +1056,15 @@ ifneq ($(CROSS_COMPILE),)
 CLANG_TARGET_ARCH = --target=$(notdir $(CROSS_COMPILE:%-=%))
 endif
 
+CLANG_OPTIONS = -Wall
 CLANG_SYS_INCLUDES = $(call get_sys_includes,$(CLANG),$(CLANG_TARGET_ARCH))
 BPF_INCLUDE := -I$(SKEL_TMP_OUT)/.. -I$(LIBBPF_INCLUDE) $(CLANG_SYS_INCLUDES)
 TOOLS_UAPI_INCLUDE := -I$(srctree)/tools/include/uapi
 
+ifneq ($(WERROR),0)
+  CLANG_OPTIONS += -Werror
+endif
+
 $(BPFTOOL): | $(SKEL_TMP_OUT)
 	$(Q)CFLAGS= $(MAKE) -C ../bpf/bpftool \
 		OUTPUT=$(SKEL_TMP_OUT)/ bootstrap
@@ -1101,7 +1106,7 @@ else
 endif
 
 $(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h | $(SKEL_TMP_OUT)
-	$(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf -Wall -Werror $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
+	$(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf $(CLANG_OPTIONS) $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
 	  -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@
 
 $(SKEL_OUT)/%.skel.h: $(SKEL_TMP_OUT)/%.bpf.o | $(BPFTOOL)