knowhere support riscv

[lyd1992]

This update introduces RISC-V 64 architecture recognition in the build system by adding dedicated macro detection (__riscv) to the CMake configuration file, while improving error diagnostics to clearly indicate riscv64 as a supported platform. These modifications ensure proper architecture validation and prevent accidental builds on unsupported platforms, laying foundational support for future RISC-V-specific optimizations like SIMD implementations.

[mergify]

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[alexanderguzhva]

/lgtm
/approve

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alexanderguzhva, lyd1992

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[alexanderguzhva]

issue: #1161

[alexanderguzhva]

/kind improvement