This macro pushes a 64-bit argument for a 64-bit call. The macro
    $SETUP_CALL64 must be used before you can use $PUSH_ARG64.

    Arguments will be read as aligned quadwords. That is, $PUSH_ARG64
    4(R0) will read the quadword at 4(R0), and push the quadword. Any
    indexed operations will be done in quadword mode.

    To push a longword value from memory as a quadword, first move it
    into a register with a longword instruction, and then use $PUSH_
    ARG64 on the register. Similarly, to push a quadword value that
    you know is not aligned, move it to a temporary register first,
    and then use $PUSH_ARG64.

    If the call contains more than six arguments on OpenVMS Alpha or
    eight arguments on OpenVMS I64, this macro checks for SP or AP
    references in the argument.

    If the call contains more than six arguments on OpenVMS Alpha or
    eight arguments on OpenVMS I64, SP references are not allowed,
    and AP references are allowed only if the inline option is used.

    Note that $PUSH_ARG64 cannot be in conditional code. $PUSH_
    ARG64 updates several symbols, such as the remaining argument
    count. Attempting to write code that branches around a $PUSH_
    ARG64 in the middle of a $SETUP_CALL64/$CALL64 sequence will not
    work properly.