selftests/bpf: Pass sanitizer flags to linker through LDFLAGS

When adding -fsanitize=address to SAN_CFLAGS, it has to be passed both
to compiler through CFLAGS as well as linker through LDFLAGS. Add
SAN_CFLAGS into LDFLAGS to allow building selftests with ASAN.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Hengqi Chen <hengqi.chen@gmail.com>
Link: https://lore.kernel.org/bpf/20211107165521.9240-2-andrii@kernel.org

tools/testing/selftests/bpf/Makefile

@@ -24,6 +24,7 @@ SAN_CFLAGS	?=
 CFLAGS += -g -O0 -rdynamic -Wall $(GENFLAGS) $(SAN_CFLAGS)		\
 	  -I$(CURDIR) -I$(INCLUDE_DIR) -I$(GENDIR) -I$(LIBDIR)		\
 	  -I$(TOOLSINCDIR) -I$(APIDIR) -I$(OUTPUT)
+LDFLAGS += $(SAN_CFLAGS)
 LDLIBS += -lcap -lelf -lz -lrt -lpthread
 
 # Silence some warnings when compiled with clang