selftests/bpf: Split SAN_CFLAGS and SAN_LDFLAGS

Memory Sanitizer requires passing different options to CFLAGS and
LDFLAGS: besides the mandatory -fsanitize=memory, one needs to pass
header and library paths, and passing -L to a compilation step
triggers -Wunused-command-line-argument. So introduce a separate
variable for linker flags. Use $(SAN_CFLAGS) as a default in order to
avoid complicating the ASan usage.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230210001210.395194-4-iii@linux.ibm.com

tools/testing/selftests/bpf/Makefile

@@ -22,10 +22,11 @@ endif
 
 BPF_GCC		?= $(shell command -v bpf-gcc;)
 SAN_CFLAGS	?=
+SAN_LDFLAGS	?= $(SAN_CFLAGS)
 CFLAGS += -g -O0 -rdynamic -Wall -Werror $(GENFLAGS) $(SAN_CFLAGS)	\
 	  -I$(CURDIR) -I$(INCLUDE_DIR) -I$(GENDIR) -I$(LIBDIR)		\
 	  -I$(TOOLSINCDIR) -I$(APIDIR) -I$(OUTPUT)
-LDFLAGS += $(SAN_CFLAGS)
+LDFLAGS += $(SAN_LDFLAGS)
 LDLIBS += -lelf -lz -lrt -lpthread
 
 # Silence some warnings when compiled with clang