RFR: 8343121: RISC-V: More accurate max size for C2SafepointPollStub and C2EntryBarrierStub

[Fei Yang]

Hi, please review this small change.

The current max size these two stubs is a bit overestimated and thus is more than needed.
Since `la`, `far_call` and `far_jump` assember routines used by these two stubs will always
emit 2 instructions for address inside the code cache, we can make the max size more accurate.

Testing on linux-riscv64 platform:
- [x] tier1-tier3 (release)
- [x] hotspot:tier1 (fastdebug)

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Commit messages:
 - 8343121: RISC-V: More accurate max size for C2SafepointPollStub and C2EntryBarrierStub

Changes: https://git.openjdk.org/jdk/pull/21732/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21732&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343121
  Stats: 7 lines in 1 file changed: 3 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21732.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21732/head:pull/21732

PR: https://git.openjdk.org/jdk/pull/21732