[compiler-rt] [test] Consume stderr from the subprocess for getting page size

This avoids dozens of instances of benign error messages being
printed when running the tests on e.g. Windows:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: module 'os' has no attribute 'sysconf'

Author: mstorsjo

compiler-rt/test/lit.common.cfg.py

@@ -970,6 +970,7 @@ def target_page_size():
             shell=True,
             stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
         )
         out, err = proc.communicate(b'import os; print(os.sysconf("SC_PAGESIZE"))')
         return int(out)