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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> Wouldn't it be better to just fix the offending script to not use sysconf on Windows?

I guess that could work as well, although the script being inline in the `proc.communicate(b'import os; print(os.sysconf("SC_PAGESIZE"))')`, so adding conditionals in that makes it a bit more messy (do conditionals work in such indentation-free python?).

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