selftests/bpf: compile progs with -D__TARGET_ARCH_$(SRCARCH)

This opens up the possibility of accessing registers in an
arch-independent way.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/Makefile

 # SPDX-License-Identifier: GPL-2.0
 include ../../../../scripts/Kbuild.include
+include ../../../scripts/Makefile.arch
 
 LIBDIR := ../../../lib
 BPFDIR := $(LIBDIR)/bpf
@@ -139,7 +140,8 @@ CLANG_SYS_INCLUDES := $(shell $(CLANG) -v -E - </dev/null 2>&1 \
 
 CLANG_FLAGS = -I. -I./include/uapi -I../../../include/uapi \
 	      $(CLANG_SYS_INCLUDES) \
-	      -Wno-compare-distinct-pointer-types
+	      -Wno-compare-distinct-pointer-types \
+	      -D__TARGET_ARCH_$(SRCARCH)
 
 $(OUTPUT)/test_l4lb_noinline.o: CLANG_FLAGS += -fno-inline
 $(OUTPUT)/test_xdp_noinline.o: CLANG_FLAGS += -fno-inline