Add RISC-V 64-bit (riscv64) platform support

[gounthar]

The next-gen download broker does not recognize LINUX_RISCV64 as a platform, preventing riscv64 users from downloading SDK candidates.

Changes needed

1. Platform.kt: add LinuxRISCV64 data object with code "linuxriscv64", persistentId "LINUX_RISCV64", auditId "LINUX_RISCV64", and add it to platformMap
2. TargetTriple.kt: add LINUX_RISCV64("riscv64gc-unknown-linux-gnu") enum value and map Platform.LinuxRISCV64 to it

Evidence

Temurin (JDK 21-25) and Liberica (JDK 17-25) already ship riscv64 Linux JDK builds. The Disco API already serves them:

https://api.foojay.io/disco/v3.0/packages?distribution=temurin&latest=available&release_status=ga&architecture=riscv64&operating_system=linux&package_type=jdk&archive_type=tar.gz

Related

• feat: add RISC-V 64-bit platform support sdkman-cli#1496: riscv64 platform detection in the shell CLI
• feat: add RISC-V 64-bit platform support sdkman-cli-native#375: riscv64 cross-compilation for the native CLI
• feat: add RISC-V 64-bit platform support sdkman-disco-integration#46: riscv64 platform mapping in the Disco API bridge
• feat: add RISC-V 64-bit (riscv64) platform support vendor-release#17: riscv64 platform in the vendor release API

This is part of a cross-ecosystem effort to enable RISC-V 64-bit users to use SDKMAN natively.