Function encode_upcall_trd64bit 

pub fn encode_upcall_trd64bit(
    upcall: &FunctionCall,
    a0: &mut u64,
    a1: &mut u64,
    a2: &mut u64,
    a3: &mut u64,
)

Encode the upcall arguments into 4 registers compatible with TRD-RISCV64.