[riscv-port] RFR: 8278547: riscv: Refactor moving assembler functions for vector instructions into separate file

[Yanhong Zhu]

RISCV have several feature extensions: RVV, RVC and RVB.
Work in under progress to support those features in Hotspot JVM.
For maintenance purposes, we should move assembler functions for specific feature into separate file.
This patch refactors existing code moving assembler functions for RVV into feature-specific files.

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Commit messages:
 - 8278547: riscv: Refactor moving assembler functions for vector instructions into separate file

Changes: https://git.openjdk.java.net/riscv-port/pull/32/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=32&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278547
  Stats: 1349 lines in 3 files changed: 696 ins; 651 del; 2 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/32.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/32/head:pull/32

PR: https://git.openjdk.java.net/riscv-port/pull/32