[compiler-rt] [compiler-rt] [test] Consume stderr from the subprocess for getting page size (PR #168857)

[Florian Mayer via llvm-commits]

fmayer wrote:

> proc.communicate(b'import os; print(os.sysconf("SC_PAGESIZE"))

how about this?

```
% python -c 'import os; print(os.sysconf("SC_PAGESIZE") if hasattr(os, "sysconf") else "")'
4096
```

https://github.com/llvm/llvm-project/pull/168857