Generate per-invocation jmp instructions.

Generate per-invocation jmp instructions.

I hypothesize that this will help the CPU's branch target predictor
be more precise than having a single jmp instruction inside an
assembly hook that actually jumps to an unknown procedure.

Empirically, this gives about 5x speed improvement for a
microbenchmark involving unknown procedure calls:

(define x (make-vector 1))

(define (test-01 x)
  (define (g)
    (vector-set! x 0 0))
  (g)
  ((identity-procedure g)))

(define (test-10 x)
  (define (g)
    (vector-set! x 0 0))
  ((identity-procedure g))
  (g))

(define (repeat n f x)
  (show-time
   (lambda ()
     (do ((i 0 (fix:+ i 1)))
         ((fix:>= i n))
       (f x)))))

; Before:
(repeat 10000000 test-01 x)
;process time: 1420 (1370 RUN + 50 GC); real time: 1427

; After:
(repeat 10000000 test-01 x)
;process time: 290 (220 RUN + 70 GC); real time: 312

Caveat: This is on top of a bunch of other experiments.
XXX Redo this in isolation.

WARNING: This adds hooks to the amd64 compiled code interface, so new
compiled code requires a new microcode.  (However, a new microcode
should handle old compiled code just fine.)