perf build: Warn for BPF skeletons if endian mismatches

Done as a warning as I'm not fully confident of the test's robustness
of comparing the macro definition of __BYTE_ORDER__.

v2. Is a rebase following patch 1 being merged.
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230410160905.3052640-1-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.config

@@ -663,14 +663,17 @@ ifndef NO_BPF_SKEL
   $(call feature_check,clang-bpf-co-re)
   ifeq ($(feature-clang-bpf-co-re), 0)
     dummy := $(error: ERROR: BPF skeletons unsupported. clang too old/not installed or build with NO_BPF_SKEL=1.)
-  else
+  endif
   ifeq ($(filter -DHAVE_LIBBPF_SUPPORT, $(CFLAGS)),)
     dummy := $(error: ERROR: BPF skeletons unsupported. BPF skeleton support requires libbpf or build with NO_BPF_SKEL=1.)
-    else
-      $(call detected,CONFIG_PERF_BPF_SKEL)
-      CFLAGS += -DHAVE_BPF_SKEL
   endif
+  host_byte_order=$(echo ""|$(HOSTCC) -dM -E -|grep __BYTE_ORDER__)
+  target_byte_order=$(echo ""|$(CC) -dM -E -|grep __BYTE_ORDER__)
+  ifneq ($(host_byte_order), $(target_byte_order))
+    $(warning Possibly mismatched host and target endianness may break BPF skeletons)
   endif
+  $(call detected,CONFIG_PERF_BPF_SKEL)
+  CFLAGS += -DHAVE_BPF_SKEL
 endif
 
 dwarf-post-unwind := 1