split nonnmethod into compiler scratch buffer area and the code stubs

[bulasevich]

This change introduces a non-executable code segment to separate temporary compiler data buffers from executable stubs within the non-nmethod segment. Compiler data buffers could be allocated outside the code cache with os::malloc, but this would cause problems with offset limits of PC relative jumps to compiled stubs. This change improves code density and hopefully reduces branch misses and cache misses (testing in progress).

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/19262/head:pull/19262
$ git checkout pull/19262

Update a local copy of the PR:
$ git checkout pull/19262
$ git pull https://git.openjdk.org/jdk.git pull/19262/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 19262

View PR using the GUI difftool:
$ git pr show -t 19262

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/19262.diff

[bridgekeeper]

👋 Welcome back bulasevich! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

@bulasevich The following label will be automatically applied to this pull request:

• hotspot-compiler

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.