PPC: fix const pool start address calculation

After the changes done by http://crrev.com/c/7865920 we need to use
`constant_pool()` instead of `code->constant_pool()` in order to
calculate the correct CP start address.

`code->constant_pool()` depends on `code->instruction_start()` which is
no longer set in NewCode and is now deferred to the end of
`InstructionStream::Finalize()`.

Change-Id: Iab4fbfa26a2f3a3552382f41cbc505c3cba7de20
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/7885002
Commit-Queue: Milad Farazmand <mfarazma@ibm.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#107688}