RFR: Fix Optimized builds [v2]

[Aleksey Shipilev]

> The usual trap in Hotspot code is the belief that `ASSERT <=> !PRODUCT`, but that's not actually true. You would not see this in regular release (`!ASSERT`, `PRODUCT`) or fastdebug (`ASSERT`, `!PRODUCT`) builds, but you would see it in optimized (`!ASSERT`, `!PRODUCT`) builds. See for example GHA: https://github.com/shipilev/loom/runs/3717294006?check_suite_focus=true
> 
> The use of `ASSERT` and `PRODUCT` should be consistent to avoid build failures in optimized builds.
> 
> Additional testing:
>  - [x] Linux x86_64 optimized now builds
>  - [x] Linux x86_64 release still builds
>  - [x] Linux x86_64 fastdebug still builds

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - More work
 - Merge branch 'fibers' into fix-optimized-builds
 - Consistent use for ASSERT and PRODUCT

-------------

Changes:
  - all: https://git.openjdk.java.net/loom/pull/69/files
  - new: https://git.openjdk.java.net/loom/pull/69/files/9070ba6a..6ee17298

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=loom&pr=69&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=loom&pr=69&range=00-01

  Stats: 27 lines in 8 files changed: 16 ins; 3 del; 8 mod
  Patch: https://git.openjdk.java.net/loom/pull/69.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/69/head:pull/69

PR: https://git.openjdk.java.net/loom/pull/69