gn build: Unbreak get.py and gn.py on Windows

[thakis]

Summary

Inspired by the recent changes here I figured I'd test these on Windows. Looks like I haven't done that in a while, they were both broken.

os.uname() doesn't exist on Windows, so use platform.machine() which returns os.uname()[4] on non-Win and (on 64-bit systems) "AMD64" on Windows. Also use sys.platform instead of platform to check for Windows-ness for the file extension in gn.py (get.py got this right).

Using [gn, '--version'] works on Windows, but I think it does the wrong thing on POSIX where it passes --version to the shell instead of to gn, so I think after r355645 the "broken gn on PATH" case isn't working right on POSIX. I'll leave that to you to fix.

[hctim]

LGTM, but as I don't have a machine to test this on I'll leave up to @pcc. Will patch up the stringification of the subprocess call and cc you there.

[pcc]

LGTM

[thakis]

I landed the follow-up to the shell=True change in r355694 since it wasn't done yet and I had three spare minutes this morning.

llvm/utils/gn/get.py

 
 
 def get_platform():
-    if os.uname()[4] != 'x86_64':
+    import platform
+    if platform.machine() not in ('AMD64', 'x86_64'):
         return None
     if sys.platform.startswith('linux'):
         return 'linux-amd64'

llvm/utils/gn/gn.py

 
 
 def get_platform():
-    if os.uname()[4] != 'x86_64':
+    import platform
+    if platform.machine() not in ('AMD64', 'x86_64'):
         return None
     if sys.platform.startswith('linux'):
         return 'linux-amd64'

         if not platform:
             return print_no_gn(mention_get=False)
         gn = os.path.join(os.path.dirname(__file__), 'bin', platform, 'gn')
-        if not os.path.exists(gn + ('.exe' if platform == 'windows' else '')):
+        if not os.path.exists(gn + ('.exe' if sys.platform == 'win32' else '')):
             return print_no_gn(mention_get=True)
 
     # Compute --dotfile= and --root= args to add.