Fix getting the heap argument

First argument: rdi register by the C calling conventions (for amd64).

scm/inc/compiler.scm

  (display ".globl scheme_entry\n\n")
  (display "scheme_entry:\n")
  (emit "movq %rax, %rsi") ; store pointer to heap memory
  (emit "movq %rdi, %rsi") ; store pointer to heap memory
  (emit-expr x (- wordsize) '())
  (emit "ret"))