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

[gounthar]

The legacy download broker does not recognize LINUX_RISCV64 as a platform.

Changes needed

1. Platform.java: add LINUX_RISCV64(newArrayList("LinuxRISCV64")) enum value
2. NativeTarget.java: add LINUX_RISCV64("riscv64gc-unknown-linux-gnu", Platform.LINUX_RISCV64) and case statement

Related

• feat: add RISC-V 64-bit platform support sdkman-cli#1496, feat: add RISC-V 64-bit platform support sdkman-cli-native#375, feat: add RISC-V 64-bit platform support sdkman-disco-integration#46, feat: add RISC-V 64-bit (riscv64) platform support vendor-release#17, feat: add RISC-V 64-bit (riscv64) platform support sdkman-broker-2#10