[riscv-port-jdk11u:riscv-port] RFR: 8291893: riscv: remove fence.i used in user space

[Gui Cao]

Please help review this backport to riscv-port-jdk11u.
Backport of [JDK-8291893](https://bugs.openjdk.org/browse/JDK-8291893)

In addition to the backport [8291947](https://bugs.openjdk.org/browse/JDK-8291947), we can use __builtin__clear_cache instead of icache_flush. 

Testing:

- [x] Run tier1-3 tests on SOPHON SG2042 (release)

-------------

Commit messages:
 - 8310656: RISC-V: __builtin___clear_cache can fail silently.
 - Merge remote-tracking branch 'upstream/riscv-port' into backport-JDK-8291893
 - Backport 8291893: riscv: remove fence.i used in user space

Changes: https://git.openjdk.org/riscv-port-jdk11u/pull/16/files
  Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=16&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291893
  Stats: 194 lines in 12 files changed: 133 ins; 53 del; 8 mod
  Patch: https://git.openjdk.org/riscv-port-jdk11u/pull/16.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk11u.git pull/16/head:pull/16

PR: https://git.openjdk.org/riscv-port-jdk11u/pull/16