[riscv-port-jdk17u:riscv-port] RFR: 8291893: riscv: remove fence.i used in user space

[Dingli Zhang]

Please review this backport to riscv-port-jdk17u.
Backport of [JDK-8291893](https://bugs.openjdk.org/browse/JDK-8291893).

The original patch cannot be directly applied because of the line number, but there are no other additional changes.

Testing:
- Tier1 passed without new failure on unmacthed (release).

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Commit messages:
 - 8291893: riscv: remove fence.i used in user space

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/13/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=13&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291893
  Stats: 77 lines in 10 files changed: 8 ins; 63 del; 6 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/13.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u pull/13/head:pull/13

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/13