Create a new function NewHeapNumberForCodeAssembler used only by
CodeAssembler that allocates in read_only_space whenever it is writable
(e.g. when constructing builtins). This can allocate in old_space for
CSA tests that run after read_only_space is sealed.

This move 512 bytes from old_space to read_only_space.

Bug: v8:7464
Change-Id: I3e29b38a8c062fa74feed63ecf0d07625c04347f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1752855
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63220}