Looing at `PCBuild\_ctypes.vcproj`, `win32.c` should be considered only for 32-bit build whereas `win64.asm` should be considered only for 64-bit build.

For 64-bit Debug, the custom command is:

```
Name="Debug|x64"
CommandLine="ml64 /nologo /c /Zi /Fo &quot;$(IntDir)\win64.obj&quot; &quot;$(InputPath)&quot;&#x0D;&#x0A;"
Outputs="$(IntDir)\win64.obj"
```

For 64-bit Release, the custom command is:
```
Name="Release|x64"
CommandLine="ml64 /nologo /c /Fo &quot;$(IntDir)\win64.obj&quot; &quot;$(InputPath)&quot;&#x0D;&#x0A;"
Outputs="$(IntDir)\win64.obj"
```