RFR: 8290496: riscv: Fix build warnings-as-errors with GCC 11

[Fei Yang]

Like AArch64 port, RISC-V port defines DEOPTIMIZE_WHEN_PATCHING and does not make
use of C1 runtime patching. Then there is no need to implement class NativeMovRegMem
in the port-specific code. But that will make GCC 11 unhappy.

One way would be guarding the C1 shared code like class PatchingStub which uses class
NativeMovRegMem under macro DEOPTIMIZE_WHEN_PATCHING. But turns out class PatchingStub
are still partially used (mainly the PatchID enum) even under DEOPTIMIZE_WHEN_PATCHING.

Like AArch64 port, this PR simply workaround this issue by implementing the NativeMovRegMem
class for RISC-V port.

Testing: release/fastdebug builds without --disable-warnings-as-errors

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Commit messages:
 - 8290496: riscv: Fix build warnings-as-errors with GCC 11

Changes: https://git.openjdk.org/jdk/pull/9550/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9550&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290496
  Stats: 43 lines in 2 files changed: 6 ins; 14 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/9550.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9550/head:pull/9550

PR: https://git.openjdk.org/jdk/pull/9550