This macro initializes the state for a 64-bit call. It must be
    used before using $PUSH_ARG64 and $CALL64.

    If there are six or fewer arguments on OpenVMS Alpha, or eight or
    fewer on OpenVMS I64, the code is always in line.

    By default, if there are more than six arguments on OpenVMS
    Alpha or eight arguments on OpenVMS I64, this macro creates a
    JSB routine that is invoked to perform the actual call. However,
    if the inline option is specified as inline=true, the code is
    generated in line.

    This option should be enabled only if the code in which it
    appears has a fixed stack depth. A fixed stack depth can be
    assumed if no RUNTIMSTK or VARSIZSTK messages have been reported.
    Otherwise, if the stack alignment is not at least quadword,
    there might be many alignment faults in the called routine
    and in anything the called routine calls. The default behavior
    (inline=false) does not have this problem.

    If there are more than six arguments on OpenVMS Alpha or eight
    arguments on OpenVMS I64, there can be no references to AP or
    SP between a $SETUP_CALL64 and the matching $CALL64, because
    the $CALL64 code may be in a separate JSB routine. In addition,
    temporary registers (R16 and above) may not survive the $SETUP_
    CALL64. However, they can be used within the range, except where
    (on Alpha only) R16 through R21 interfere with the argument
    registers already set up. In such cases, higher temporary
    registers should be used instead.

                                   NOTE

       The $SETUP_CALL64, $PUSH_ARG64, and $CALL64 macros are
       intended to be used in an inline sequence. That is, you
       cannot branch into the middle of a $SETUP_CALL64/$PUSH_
       ARG64/$CALL64 sequence, nor can you branch around $PUSH_
       ARG64 macros or branch out of the sequence to avoid the
       $CALL64.