selftests/bpf: Pass sanitizer flags to linker through LDFLAGS

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