RFR: 8294468: Fix char-subscripts warnings in Hotspot

[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}

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

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/10455/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10455&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294468
  Stats: 4 lines in 2 files changed: 1 ins; 0 del; 3 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