RFR: 8243208: Clean up JVMFlag implementation [v6]

[Ioi Lam]

> The initial commit [d85cbd6](https://github.com/openjdk/jdk/commit/d85cbd638f5b87c80eee9a92d10e1ac19d9bb2f8) is the
> same patch as
> [8243208-cleanup-jvmflag-impl.v02](https://cr.openjdk.java.net/~iklam/jdk16/8243208-cleanup-jvmflag-impl.v02/)
> published in
> [hotspot-dev at openjdk.java.net](https://mail.openjdk.java.net/pipermail/hotspot-dev/2020-September/042863.html).  The
> rest of the review will continue on GitHub. I will add new commits to respond to comments to the above e-mail.

Ioi Lam 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 seven additional commits since the last
revision:

 - Merge branch 'master' into 8243208-cleanup-jvmflag-impl
 - Minor code clean up suggested by David Holmes
 - Feedback by Gerard (hotspot-dev/2020-September/043019.html)
 - Refactored JVMFlagLookup::hash_code(); Cleaned up LimitGetter::no_limit()
 - Fixed JVMFlag::is_writeable()
 - (a) coding style nits and comments, (b) updated HOWTO in globals.hpp, (c) added JVMFlag::check_all_flag_declarations(),
   (d) removed product_rw which is not used by any flags
 - 8243208: Clean up JVMFlag implementation

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/82/files
  - new: https://git.openjdk.java.net/jdk/pull/82/files/15547ebe..1b63ac6f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=82&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=82&range=04-05

  Stats: 452 lines in 56 files changed: 165 ins; 165 del; 122 mod
  Patch: https://git.openjdk.java.net/jdk/pull/82.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/82/head:pull/82

PR: https://git.openjdk.java.net/jdk/pull/82