[jdk21u] RFR: 8319525: RISC-V: Rename *_riscv64.ad files to *_riscv.ad under riscv/gc

[Fei Yang]

Clean backport to make possible future backports to those AD files smooth.

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Commit messages:
 - Backport b2504a0f9cff1f4fdf8f5bb20c4b48a53ba0be77

Changes: https://git.openjdk.org/jdk21u/pull/338/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=338&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/jdk21u/pull/338.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/338/head:pull/338

PR: https://git.openjdk.org/jdk21u/pull/338