fix: support riscv64gc bindgen target

[fanyang89]

Summary

• map Rust's riscv64gc target triple to clang-compatible riscv64 for bindgen
• pass rv64gc CPU features to bindgen via -march=rv64gc
• add riscv64gc-unknown-linux-musl to the zigbuild CI matrix

Verification

• cargo fmt --check
• cargo check
• cargo zigbuild --target riscv64gc-unknown-linux-musl --release
• EasyTier: cargo zigbuild --release --target riscv64gc-unknown-linux-musl --package=easytier --features=mimalloc

[Copilot]

Pull request overview

This PR improves cross-compilation support for the Rust riscv64gc target by ensuring bindgen/libclang is invoked with a clang-compatible target triple and appropriate RISC-V ISA flags, and extends CI to build the new target via cargo-zigbuild.

Changes:

• Normalize riscv64gc to riscv64 for clang/bindgen target handling.
• Pass -march=rv64gc to bindgen when targeting riscv64gc-*.
• Add riscv64gc-unknown-linux-musl (debug/release) to the zigbuild CI matrix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
build.rs	Adds clang-target normalization and target-specific clang args for bindgen, including RISC-V -march.
.github/workflows/rust.yml	Expands cross-compilation matrix to include riscv64gc-unknown-linux-musl builds.

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