.. and add a --text-is-readable flag to support non-readable .text
sections.

This splits the embedded blob hash into two dedicated hashes for data
and code sections. The main benefit is that we can now keep at least a
partial hash even with non-readable .text sections.

The second part of this CL adds a --text-is-readable runtime flag to
support such platforms (with non-readable .text).

It currently doesn't do much; setting it enables a few additional
DCHECKs, disables the constant pool on x64, and and disables
verification of the embedded blob's *code* hash.

Bug: v8:10707
Change-Id: Ib91ed8b50b50f2cd81677f62920bea6fb92af453
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2504251Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70827}