RFR: 8295711: Rename ZBarrierSetAssembler::load_at parameter name from "tmp_thread" to "tmp2"

[Fei Yang]

On AArch64 and RISC-V, the last formal parameter for ZBarrierSetAssembler::load_at
is named "tmp_thread". But the callers will pass an ordinary temporary register for
this parameter which has no relation with the thread register. We should rename this
formal parameter from "tmp_thread" to "tmp2".

Testing: Fastdebug builds on linux-aarch64 & linux-riscv64.

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Commit messages:
 - 8295711: Rename ZBarrierSetAssembler::load_at parameter name from "tmp_thread" to "tmp2"

Changes: https://git.openjdk.org/jdk/pull/10783/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10783&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295711
  Stats: 6 lines in 4 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/10783.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10783/head:pull/10783

PR: https://git.openjdk.org/jdk/pull/10783