Fix Ninja support (#24584)

Fixes #24393 by using subprocess.list2cmdline instead of shlex.join.

Unfortunately, subprocess.list2cmdline is undocumented but apparently
has been available for well over a decade (eg see
https://stackoverflow.com/questions/12130163/how-to-get-resulting-subprocess-command-string)
and, judging by discussions like
https://discuss.python.org/t/why-is-subprocess-list2cmdline-not-documented/25044
is unlikely to be ever removed since it's been long used in the wild.

It does exactly what we want - quote & join a command by Windows rules.

Author: RReverser

tools/system_libs.py

@@ -13,6 +13,7 @@
 import shutil
 import textwrap
 import shlex
+import subprocess
 from enum import IntEnum, auto
 from glob import iglob
 from typing import List, Optional
@@ -183,13 +184,15 @@ def create_ninja_file(input_files, filename, libname, cflags, asflags=None, cust
   if asflags is None:
     asflags = []
 
+  join_cmd = shlex.join if os.name == 'posix' else subprocess.list2cmdline
+
   out = f'''\
 # Automatically generated by tools/system_libs.py.  DO NOT EDIT
 
 ninja_required_version = 1.5
 
-ASFLAGS = {shlex.join(asflags)}
-CFLAGS = {shlex.join(cflags)}
+ASFLAGS = {join_cmd(asflags)}
+CFLAGS = {join_cmd(cflags)}
 EMCC = {shared.EMCC}
 EMXX = {shared.EMXX}
 EMAR = {shared.EMAR}
@@ -267,7 +270,7 @@ def create_ninja_file(input_files, filename, libname, cflags, asflags=None, cust
       if customize_build_flags:
         custom_flags = customize_build_flags(flags, src)
         if custom_flags != flags:
-          out += f'  CFLAGS = {shlex.join(custom_flags)}'
+          out += f'  CFLAGS = {join_cmd(custom_flags)}'
       out += '\n'
 
     objects = sorted(objects, key=objectfile_sort_key)