[riscv-port-jdk17u:riscv-port] RFR: 8296975: RISC-V: Enable UseRVA20U64 profile by default

[Xiaolin Zheng]

The last backported patch of RVC - to enable it by default.

Although, we don't have `UseRVA20U64` in jdk17, so I only made `UseRVC` true here. Looking forward to some opinions about it.

Tested along with remaining patches, hotspot tier1~4 with fastdebug build.

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Commit messages:
 - 8296975: RISC-V: Enable UseRVA20U64 profile by default

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/33/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=33&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296975
  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/33.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/33/head:pull/33

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/33