Function ecall

#[no_mangle]
pub fn ecall(
    a0: usize,
    a1: usize,
    a2: usize,
    a3: usize,
    a4: usize,
    a5: usize,
    a6: usize,
    a7: usize,
) -> usize

RISC-V environment call (ecall) with proper register preservation using clobber_abi to handle register preservation automatically