RFR: 8364120: RISC-V: unify the usage of MacroAssembler::instruction_size

[Hamlin Li]

Hi,
Can you help to review this simple cleanup?

in MacroAssembler, some magic number 4 is used for MacroAssembler::instruction_size. We should not use magic number if possible.
This pr also replaces all the usage of  `instruction_size` with `MacroAssembler::instruction_size`.

Thanks

-------------

Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/26481/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26481&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364120
  Stats: 38 lines in 2 files changed: 0 ins; 0 del; 38 mod
  Patch: https://git.openjdk.org/jdk/pull/26481.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26481/head:pull/26481

PR: https://git.openjdk.org/jdk/pull/26481