RFR: 8294187: RISC-V: Unify all relocations for the backend into MacroAssembler::relocate()

[Xiaolin Zheng]

Unify all relocations into MacroAssembler::relocate() as a simple cleanup, so that there are platform-related entrances for them. We will have needs to insert some code into the new MacroAssembler::relocate() afterward. Also, the (__ pc()) everywhere seems a little verbose here.

Tested hotspot tier1 and tier2 on QEMU.

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Commit messages:
 - RISC-V: Unify all relocations for the backend to MacroAssembler::relocate()

Changes: https://git.openjdk.org/jdk/pull/10384/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10384&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294187
  Stats: 37 lines in 6 files changed: 18 ins; 6 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/10384.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10384/head:pull/10384

PR: https://git.openjdk.org/jdk/pull/10384