RFR: 8303955: RISC-V: Factor out the tmp parameter from copy_memory and copy_memory_v

[Feilong Jiang]

The call site of `copy_memory` and `copy_memory_v` always use `t0` as tmp register, so we can factor the tmp parameter out.

Testing:

- [x] tier1 tests on Unmatched board (release build with `-XX:-UseRVV`)
- [x] hotspot_tier1 and jdk_tier1 on QEMU (release build with `-XX:+UseRVV`)

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Commit messages:
 - Factor out the tmp parameter from copy_memory and copy_memory_v

Changes: https://git.openjdk.org/jdk/pull/12969/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12969&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303955
  Stats: 29 lines in 1 file changed: 0 ins; 0 del; 29 mod
  Patch: https://git.openjdk.org/jdk/pull/12969.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12969/head:pull/12969

PR: https://git.openjdk.org/jdk/pull/12969