RFR: 8314488: Compile the JDK as C++17

[Julian Waters]

Implementation of [JEP draft: Compile the JDK as C++17](https://bugs.openjdk.org/browse/JDK-8310260)

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Commit messages:
 - vm_version_linux_riscv.cpp
 - allocation.cpp
 - 8310260

Changes: https://git.openjdk.org/jdk/pull/14988/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14988&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314488
  Stats: 5 lines in 3 files changed: 0 ins; 0 del; 5 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