[riscv-port] RFR: 8277391: riscv: Implement tls function MacroAssembler::get_thread() using tls.ie

[zhengxiaolinX]

Hi team,

This patch fixes the unimplemented assembly version of `__ get_thread()`, bringing some performance improvement by directly using the x4 register to refer to the variable `Thread::_thr_current`. Tested tests under hotspot/ and jdk/. [Original patch](https://github.com/riscv-collab/riscv-openjdk/pull/4)

Thanks,
Xiaolin

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Commit messages:
 - Implement tls function MacroAssembler::get_thread() using tls.ie

Changes: https://git.openjdk.java.net/riscv-port/pull/7/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=7&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277391
  Stats: 60 lines in 3 files changed: 44 ins; 12 del; 4 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/7.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/7/head:pull/7

PR: https://git.openjdk.java.net/riscv-port/pull/7