[riscv-port-jdk11u:riscv-port] RFR: 8294187: RISC-V: Unify all relocations for the backend into AbstractAssembler::relocate()

[Dingli Zhang]

Hi all, 

Please review this backport to riscv-port-jdk11u.

Backport of [JDK-8294187](https://bugs.openjdk.org/browse/JDK-8294187).

The original patch cannot be directly applied because jdk11 has no `C1SafepointPoll` or `C2SafepointPollStubTable`.

Testing:
- Tier1 passed without new failure on lp4a (release).

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Commit messages:
 - 8294187: RISC-V: Unify all relocations for the backend

Changes: https://git.openjdk.org/riscv-port-jdk11u/pull/39/files
  Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=39&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294187
  Stats: 31 lines in 5 files changed: 3 ins; 18 del; 10 mod
  Patch: https://git.openjdk.org/riscv-port-jdk11u/pull/39.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk11u.git pull/39/head:pull/39

PR: https://git.openjdk.org/riscv-port-jdk11u/pull/39