Use CALL/RET for pushing and returning to continuations on amd64.

Use CALL/RET for pushing and returning to continuations on amd64.

Calls now look like:

  ;; (assign (register #x123) (cons-pointer tag (entry:continuation cont)))
  (CALL (@PCR pushed))
  (JMP (@PCR cont))
pushed:
  (OR Q (@R ,rsp) (&U ,tag))
  ...
  (JMP (@PCR uuo-link))

Returns now look like:

  ;; (pop-return)
  (AND Q (@R ,rsp) (R ,regnum:datum-mask))
  (RET)

These should happen in pairs, so that we can take advantage of the
CPU's return address branch target prediction stack rather than
abusing the indirect jump branch target predictor.

WARNING: This changes the amd64 compiled code interface, so new
compiled code requires a new microcode.  (A new microcode might be
able to handle existing compiled code just fine.)