benches: Introduce riscv64 architecture

As clippy command in our CI mandates: `cargo clippy --workspace --bins
--examples --benches --all-features --all-targets -- -D warnings -D
clippy::undocumented_unsafe_blocks`, add benchmarck test to appease
clippy.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

Author: RuoqingHe

benches/main.rs

@@ -16,9 +16,9 @@ mod x86_64;
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 use x86_64::*;
 
-#[cfg(target_arch = "aarch64")]
+#[cfg(any(target_arch = "aarch64", target_arch = "riscv64"))]
 mod fdt;
-#[cfg(target_arch = "aarch64")]
+#[cfg(any(target_arch = "aarch64", target_arch = "riscv64"))]
 pub use fdt::*;
 
 criterion_group! {