feat: add RISC-V 64-bit (riscv64) platform detection

[chloe41427]

RISC-V 64-bit Platform Detection

Add LINUX_RISCV64 platform detection to the native CLI so that users running SDKMAN! on RISC-V 64-bit hardware are correctly identified and served the right candidate versions.

Background

Several community contributions have been submitted requesting RISC-V 64-bit support. These were deferred until the backend migration is complete. This issue tracks the client-side detection work needed in sdkman-cli-native.

Acceptance Criteria

• LINUX_RISCV64 platform variant added to the platform detection logic
• Correctly detected on riscv64 Linux hardware
• Integration tested with the new backend (sdkman-state)

Notes

• Coordinate with sdkman-state for the server-side platform support
• Block on backend migration being stable before implementing

[chloe41427]

Related: sdkman/sdkman-state#23 — server-side platform support for RISC-V 64-bit.