RFR: 8294468: Fix char-subscripts warnings in Hotspot [v2]

[Aleksey Shipilev]

> There seem to be the only place in Hotspot where this warning fires, yet the warning is disabled wholesale for Hotspot. This is not good.
> 
> I can trace the addition of char-subscripts exclusion to [JDK-8211029](https://bugs.openjdk.org/browse/JDK-8211029) (Sep 2018). The only place in Hotspot where in fires is present from the initial load (2007).
> 
> The underlying problem that this warning tells us about is that `char` might be signed on some platforms, so we can potentially access the negative index. It is not a bug in our current code, that bounds the value of `k` under `MAXID-1`, which is `19`.
> 
> Additional testing:
>  - [ ] Linux x86_64 fastdebug `tier1`
>  - [x] The build matrix of:
>    - GCC 10
>    - {i686, x86_64, aarch64, powerpc64le, s390x, armhf, riscv64}
>    - {server}
>    - {release, fastdebug}

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge branch 'master' into JDK-8294468-warning-char-subscripts
 - Fix

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Changes: https://git.openjdk.org/jdk/pull/10455/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10455&range=01
  Stats: 4 lines in 2 files changed: 1 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10455.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10455/head:pull/10455

PR: https://git.openjdk.org/jdk/pull/10455