RFR: 8298088: RISC-V: Make Address a discriminated union internally

[Fei Yang]

The RISC-V port has the same issue as: https://bugs.openjdk.org/browse/JDK-8297830
It looks to me that the fix for the AArch64 port is a nice refactoring work.
This fixes this issue for the RISC-V port with a similar approach.

Testing:
  Tier1 tested with release build on linux-riscv64 unmatched board.
  Run non-trivial benchmark workloads with fastdebug builds.

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Commit messages:
 - 8298088: RISC-V: Make Address a discriminated union internally

Changes: https://git.openjdk.org/jdk/pull/11505/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11505&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298088
  Stats: 142 lines in 2 files changed: 93 ins; 11 del; 38 mod
  Patch: https://git.openjdk.org/jdk/pull/11505.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11505/head:pull/11505

PR: https://git.openjdk.org/jdk/pull/11505