selftests/bpf: Makefile fix "missing" headers on build with -idirafter

Selftests fail to build on several distros/architectures because of
	missing headers files.

On a Ubuntu/x86_64 some missing headers are:
	asm/byteorder.h, asm/socket.h, asm/sockios.h

On a Debian/arm32 build already fails at sys/cdefs.h

In both cases, these already exist in /usr/include/<arch-specific-dir>,
but Clang does not include these when using '-target bpf' flag,
since it is no longer compiling against the host architecture.

The solution is to:

- run Clang without '-target bpf' and extract the include chain for the
current system

- add these to the bpf build with '-idirafter'

The choice of -idirafter is to catch this error without injecting
unexpected include behavior: if an arch-specific tree is built
for bpf in the future, this will be correctly found by Clang.
Signed-off-by: Sirio Balmelli <sirio@b-ad.ch>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/Makefile

@@ -84,7 +84,17 @@ else
   CPU ?= generic
 endif
 
+# Get Clang's default includes on this system, as opposed to those seen by
+# '-target bpf'. This fixes "missing" files on some architectures/distros,
+# such as asm/byteorder.h, asm/socket.h, asm/sockios.h, sys/cdefs.h etc.
+#
+# Use '-idirafter': Don't interfere with include mechanics except where the
+# build would have failed anyways.
+CLANG_SYS_INCLUDES := $(shell $(CLANG) -v -E - </dev/null 2>&1 \
+	| sed -n '/<...> search starts here:/,/End of search list./{ s| \(/.*\)|-idirafter \1|p }')
+
 CLANG_FLAGS = -I. -I./include/uapi -I../../../include/uapi \
+	      $(CLANG_SYS_INCLUDES) \
 	      -Wno-compare-distinct-pointer-types
 
 $(OUTPUT)/test_l4lb_noinline.o: CLANG_FLAGS += -fno-inline