[jdk17u-dev] RFR: 8319525: RISC-V: Rename *_riscv64.ad files to *_riscv.ad under riscv/gc

[Feilong Jiang]

I would like to backport [JDK-8319525](https://bugs.openjdk.org/browse/JDK-8319525) to make possible future backports to those AD files smooth.
Applies not clean since 17u does not support generational-zgc yet.

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Commit messages:
 - Backport b2504a0f9cff1f4fdf8f5bb20c4b48a53ba0be77

Changes: https://git.openjdk.org/jdk17u-dev/pull/1954/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1954&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319525
  Stats: 0 lines in 2 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1954.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1954/head:pull/1954

PR: https://git.openjdk.org/jdk17u-dev/pull/1954