Use wordsize instead of hard-coded number

scm/inc/compiler.scm

     (emit-expr (primcall-operand1 x) si env)
     (emit "movl %eax, 0(%rsi)") ; set the car
     (emit-expr (primcall-operand2 x) si env)
     (emit "movl %eax, 4(%rsi)") ; set the cdr
     (emit "movl %eax, ~a(%rsi)" wordsize) ; set the cdr
     (emit "movq %rsi, %rax") ; rax = rsi | 1  (cons cell/pair tag)
     (emit "orq  $~a, %rax" #b001)
     (emit "addq $8,  %rsi")) ; bump rsi