RFR: 8308765: RISC-V: Expand the size of stub routines when zgc only

[Yadong Wang]

Generational zgc expanded the stub size after https://bugs.openjdk.org/browse/JDK-8307058. It should only affect zgc on riscv, same as that on aarch64 and x86 port.
This PR also guarantees that li16u instructions can only accept 16 bits immediate values when patched or encoded.

The new size of stubs are tested with -XX:+UseZGC -XX:+ZGenerational -XX:-UseRVC, and checked with -Xlog:stubs.

Tier 1~3 passed on the unmatched boards.

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Commit messages:
 - 8308765: RISC-V: Expand the size of stub routines when zgc only

Changes: https://git.openjdk.org/jdk/pull/14203/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14203&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308765
  Stats: 9 lines in 3 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/14203.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14203/head:pull/14203

PR: https://git.openjdk.org/jdk/pull/14203