bpf: Move -Wno-compare-distinct-pointer-types to BPF_CFLAGS

Clang supports enabling/disabling certain conversion diagnostics via
the -W[no-]compare-distinct-pointer-types command line options.
Disabling this warning is required by some BPF selftests due to
-Werror.  Until very recently GCC would emit these warnings
unconditionally, which was a problem for gcc-bpf, but we added support
for the command-line options to GCC upstream [1].

This patch moves the -Wno-cmopare-distinct-pointer-types from
CLANG_CFLAGS to BPF_CFLAGS in selftests/bpf/Makefile so the option
is also used in gcc-bpf builds, not just in clang builds.

Tested in bpf-next master.
No regressions.

  [1] https://gcc.gnu.org/pipermail/gcc-patches/2023-August/627769.htmlSigned-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240130113624.24940-1-jose.marchesi@oracle.com

tools/testing/selftests/bpf/Makefile

@@ -404,11 +404,11 @@ endif
 CLANG_SYS_INCLUDES = $(call get_sys_includes,$(CLANG),$(CLANG_TARGET_ARCH))
 BPF_CFLAGS = -g -Wall -Werror -D__TARGET_ARCH_$(SRCARCH) $(MENDIAN)	\
 	     -I$(INCLUDE_DIR) -I$(CURDIR) -I$(APIDIR)			\
-	     -I$(abspath $(OUTPUT)/../usr/include)
+	     -I$(abspath $(OUTPUT)/../usr/include)			\
+	     -Wno-compare-distinct-pointer-types
 # TODO: enable me -Wsign-compare
 
-CLANG_CFLAGS = $(CLANG_SYS_INCLUDES) \
-	       -Wno-compare-distinct-pointer-types
+CLANG_CFLAGS = $(CLANG_SYS_INCLUDES)
 
 $(OUTPUT)/test_l4lb_noinline.o: BPF_CFLAGS += -fno-inline
 $(OUTPUT)/test_xdp_noinline.o: BPF_CFLAGS += -fno-inline