selftests/bpf: Forward SAN_CFLAGS and SAN_LDFLAGS to runqslower and libbpf

To get useful results from the Memory Sanitizer, all code running in a
process needs to be instrumented. When building tests with other
sanitizers, it's not strictly necessary, but is also helpful.
So make sure runqslower and libbpf are compiled with SAN_CFLAGS and
linked with SAN_LDFLAGS.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230210001210.395194-5-iii@linux.ibm.com