Ensured that 32-bit targets use aligned pointers for 8-byte fields.

On 32-bit targets (including WASM), the base message pointer was aligned to 4 instead of 8, causing reads to 8-byte fields to fail, since TypedArray does not support unaligned reads.

The pointer was 4-byte aligned because upb adds the size of its "internal" pointer before returning the `upb_Message*`.  We should probably stop doing this, and instead have the MiniTable offsets reflect their full and true offset from the pointer returned by `malloc()`.

PiperOrigin-RevId: 552486609

upb/message/internal.h

@@ -86,7 +86,13 @@ typedef struct {
 } upb_Message_InternalData;
 
 typedef struct {
-  upb_Message_InternalData* internal;
+  union {
+    upb_Message_InternalData* internal;
+
+    // Force 8-byte alignment, since the data members may contain members that
+    // require 8-byte alignment.
+    double d;
+  };
   /* Message data follows. */
 } upb_Message_Internal;