[riscv-port-jdk17u:riscv-port] RFR: 8301036: RISC-V: Factor out functions baseOffset & baseOffset32 from MacroAssembler

[Dingli Zhang]

Hi, please review this backport to riscv-port-jdk17u.
Backport of [JDK-8301036](https://bugs.openjdk.org/browse/JDK-8301036).
The original patch cannot be directly applied because of the missing whitespace between `offset` and `=`:
https://github.com/openjdk/riscv-port-jdk17u/blob/9b88e8e65771a7022b2ce3d7cd89281dea9ce975/src/hotspot/cpu/riscv/macroAssembler_riscv.hpp#L889

Testing:

Tier1-3 passed without new failure on unmacthed (release).

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Commit messages:
 - 8301036: RISC-V: Factor out functions baseOffset & baseOffset32 from MacroAssembler

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/54/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=54&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301036
  Stats: 64 lines in 3 files changed: 15 ins; 31 del; 18 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/54.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/54/head:pull/54

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/54