[riscv-port-jdk11u:riscv-port] RFR: 8291893: riscv: remove fence.i used in user space [v2]

[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)

Gui Cao has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  8310656: RISC-V: __builtin___clear_cache can fail silently.

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Changes:
  - all: https://git.openjdk.org/riscv-port-jdk11u/pull/16/files
  - new: https://git.openjdk.org/riscv-port-jdk11u/pull/16/files/85db9b24..6db0c80f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=16&range=01
 - incr: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=16&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 1 del; 1 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