Pointer can be const void* in GenericWireType (#21539)

...as it had already been prior to abeba41
"[Wasm64] Initial work for get parts of embind working".  It being non-const
caused some issue for client code at
<https://git.libreoffice.org/core/+/98c42f7e961e77d7f1c02d53862e4e78ecd07653%5E!>
"Adapt to emsdk 3.1.46".

Co-authored-by: Stephan Bergmann <[email protected]>

Author: stbergmann

system/include/emscripten/val.h

@@ -207,7 +207,7 @@ union GenericWireType {
     unsigned u;
     size_t s;
     float f;
-    void* p;
+    const void* p;
   } w[2];
   double d;
   uint64_t u;