Function encode_syscall_return_trd64bit 

pub fn encode_syscall_return_trd64bit(
    syscall_return: &TRDRiscv64bitSyscallReturn,
    a0: &mut u64,
    a1: &mut u64,
    a2: &mut u64,
    a3: &mut u64,
)

Encode the system call return value into 4 registers.

This follows the encoding specified in TRD-Riscv64bit.