Changes:
- Add WasmInitExpr class which knows how to create initializer
  expressions as pairs of {type, value}. Also define a default for every
  type. Emit such pairs to a byte array with emit_init_expr().
- Add an initializer expression to every global (addGlobal() uses the
  default if the argument is absent).
- Introduce wasmI64Const();
- Update tests as needed.

Change-Id: I75ffe96604891506ad78bd3677ce1efe5e0cee07
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2851892
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74231}