[riscv-port] RFR: 8278895: riscv: Rename Riscv64 to RISCV64

[Yanhong Zhu]

Rename Riscv64 to RISCV64 for consistence.

The related tests passed on QEMU:

- test/hotspot/jtreg/compiler/intrinsics/sha/cli/TestUseSHA1IntrinsicsOptionOnUnsupportedCPU.java
- test/hotspot/jtreg/compiler/intrinsics/sha/cli/TestUseSHA256IntrinsicsOptionOnUnsupportedCPU.java
- test/hotspot/jtreg/compiler/intrinsics/sha/cli/TestUseSHA512IntrinsicsOptionOnUnsupportedCPU.java
- test/hotspot/jtreg/compiler/intrinsics/sha/cli/TestUseSHAOptionOnUnsupportedCPU.java
- test/hotspot/jtreg/compiler/intrinsics/sha/cli/testcases/GenericTestCaseForOtherCPU.java
- test/hotspot/jtreg/compiler/intrinsics/sha/cli/testcases/GenericTestCaseForUnsupportedRISCV64CPU.java
- test/hotspot/jtreg/runtime/NMT/CheckForProperDetailStackTrace.java
- test/hotspot/jtreg/runtime/ReservedStack/ReservedStackTest.java
- test/jdk/jdk/jfr/event/os/TestCPUInformation.java
- test/lib-test/jdk/test/lib/TestMutuallyExclusivePlatformPredicates.java
- test/lib/jdk/test/lib/Platform.java

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Commit messages:
 - 8278895: riscv: Rename Riscv64 to RISCV64

Changes: https://git.openjdk.java.net/riscv-port/pull/33/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=33&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278895
  Stats: 32 lines in 16 files changed: 0 ins; 1 del; 31 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/33.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/33/head:pull/33

PR: https://git.openjdk.java.net/riscv-port/pull/33