[compiler-rt] [test] refine target_page_size() in lit.common.cfg.py (NFC) (#170475)

Use mmap.PAGESIZE and fallback to os.sysconf if it is not available.
This allows to query the page size on Windows too, which has mmap but
not sysconf.

This is a pedantic change because Windows has a fixed page size of 4KiB.
I found this solution independently of #168857, and thought it might be
worth committing, since this is technically more correct.

[mmap.PAGESIZE implementation in
CPython](https://github.com/python/cpython/blob/88cd5d9850d2dc51abe43eb84198904d9870c26e/Modules/mmapmodule.c#L47)

Author: Maetveis

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

@@ -968,8 +968,17 @@ def target_page_size():
             stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
         )
+        # UNIX (except WASI) and Windows can use mmap.PAGESIZE,
+        # attempt to use os.sysconf for other targets.
         out, err = proc.communicate(
-            b'import os; print(os.sysconf("SC_PAGESIZE") if hasattr(os, "sysconf") else "")'
+            b"""
+try:
+    from mmap import PAGESIZE
+    print(PAGESIZE)
+except ImportError:
+    from os import sysconf
+    print(sysconf("SC_PAGESIZE"))
+"""
         )
         return int(out)
     except: