Skip setting seccomp filter when running with ASan.

Also add an example build target for an ASan-ified libminijail (useful
for debugging).

Bug: 28052772

Change-Id: Ib36a0303d635becaa8802dee56d486f11060ea47
diff --git a/util.h b/util.h
index 0cc1d15..b4efc2f 100644
--- a/util.h
+++ b/util.h
@@ -37,6 +37,14 @@
 #endif
 }
 
+static inline int running_with_asan() {
+#if defined(__clang__) && __has_feature(address_sanitizer)
+	return 1;
+#else
+	return 0;
+#endif
+}
+
 int lookup_syscall(const char *name);
 const char *lookup_syscall_name(int nr);
 long int parse_constant(char *constant_str, char **endptr);