Fix + on x86_64

This also means that from now on we *only* work on x86_64.

scm/inc/compiler.scm

     (emit-compare))
    ((+)
     (emit-expr (primcall-operand2 x) si)
     (emit "movl %eax, ~a(%rbp)" si) ; move second arg on the stack
     (emit "movl %eax, ~a(%rsp)" si) ; move second arg on the stack
     (emit-expr (primcall-operand1 x) (- si wordsize))
     (emit "addl ~a(%rbp), %eax" si))
     (emit "addl ~a(%rsp), %eax" si))
    ))
(define (compile-program x)