RFR: 8314488: Compile the JDK as C++17 [v3]

[Julian Waters]

> Compile the JDK as C++17, enabling the use of all C++17 language features

Julian Waters 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:

 - Compiler versions in toolchain.m4
 - Merge branch 'openjdk:master' into patch-7
 - Merge branch 'openjdk:master' into patch-7
 - Revert vm_version_linux_riscv.cpp
 - vm_version_linux_riscv.cpp
 - allocation.cpp
 - 8310260

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14988/files
  - new: https://git.openjdk.org/jdk/pull/14988/files/a1f21bbd..477f6b94

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14988&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14988&range=01-02

  Stats: 2377 lines in 158 files changed: 1669 ins; 268 del; 440 mod
  Patch: https://git.openjdk.org/jdk/pull/14988.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14988/head:pull/14988

PR: https://git.openjdk.org/jdk/pull/14988