tool.drcacheoff.rseq failing with glibc 2.35

The `tool.drcacheoff.rseq` test fails due to an invariant error, on an A64 VM with glibc 2.35 (disabled glibc's rseq due to i#5431). The same also happens with the #5711 fix, with and without glibc's rseq.

```
238: Trace invariant failure in T-1 at ref # 0: Serial schedule entry count does
238: not match trace
```

The test works fine on the A64 Jenkins machine (which has an older glibc).