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

[Feilong Jiang]

Hi, please review this trivial patch that renames `*_riscv64.ad` files to `*_riscv.ad`.
In the early development at jdk-sandbox, there was a commit [1] that renamed all filenames from riscv64 to riscv, but some `.ad` files under `riscv/gc` were missed.

1. https://github.com/openjdk/jdk-sandbox/commit/5500685c274b1b96c1dd5440b5203432081cc679


Testing
- [x] release build
- [x]  hotspot:tier1

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Commit messages:
 - rename _riscv64 suxfix into _riscv

Changes: https://git.openjdk.org/jdk/pull/16521/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16521&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319525
  Stats: 0 lines in 3 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16521.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16521/head:pull/16521

PR: https://git.openjdk.org/jdk/pull/16521