conf: Do not set -fno-sanitize-memory-param-retval for old compilers

The option was introduced in LLVM 16.

Fixes: b6c653c97463 ("kmsan: Set -fno-sanitize-memory-param-retval for now")
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.