benches: Re-organize module layout

Code in `mod.rs` of `aarch64` do not contain architecture specific code
as of `aarch64`, it's only used by `aarch64` to run benchmark tests,
move it one level up and rename it to `fdt.rs` to allow reusing on
riscv64 platform.

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

Author: RuoqingHe

benches/aarch64/mod.rs renamed to benches/fdt.rs

@@ -17,9 +17,9 @@ mod x86_64;
 use x86_64::*;
 
 #[cfg(target_arch = "aarch64")]
-mod aarch64;
+mod fdt;
 #[cfg(target_arch = "aarch64")]
-use aarch64::*;
+pub use fdt::*;
 
 criterion_group! {
     name = benches;