eclic: move eclic-mode-hack.S into Rust

Uses inline assembly to move the code from `eclic-mode-hack.S` into
Rust.

The generated assembly is different. This commit should be reverted if
regressions are noticed on real hardware.

Author: rmsyn

assemble.sh

@@ -1,5 +1,9 @@
 use crate::pac::{ECLIC, Interrupt};
 
+/// The code is based on vendor provided HAL libraries.
+/// Most code come from Firmware\RISCV\env_Eclipse\start.S
+pub mod mode;
+
 const EFFECTIVE_LEVEL_PRIORITY_BITS: u8 = 4;
 
 #[repr(u8)]