zlib: Add importC special case for va_start() on Win64

This fixes an undefined `__va_start` symbol for the zlib parts,
a problem on Win64 only so far.

The Microsoft headers use something like this (`vadefs.h`) for
x86_64:

```
void __cdecl __va_start(va_list* , ...);
```

The signature of their intrinsic isn't compatible with druntime's
`va_start` (which takes the first param as `out` ref, not as
explicit pointer), so I sadly don't see a way to handle this in
druntime's `__builtins.di` (or `importc.h`).

etc/c/zlib/gzwrite.c

@@ -465,7 +465,12 @@ int ZEXPORTVA gzprintf(gzFile file, const char *format, ...)
     va_list va;
     int ret;
 
+// needed on Win64 - MS __va_start intrinsic not supported by importC yet
+#if __IMPORTC__
+    __builtin_va_start(va, format);
+#else
     va_start(va, format);
+#endif
     ret = gzvprintf(file, format, va);
     va_end(va);
     return ret;