You can invoke this macro to locate or define a linkage pair
    in the linkage psect. The linkage pair is added to a list for
    later use and retrieval by the $CALL and $LINKAGE_PAIR macros.
    Thus, only the first invocation of $LINKAGE_PAIR (or $CALL) for
    a given routine to be called results in a linkage pair being
    stored. $LINKAGE_PAIR and $CALL use the same list of linkage
    pairs. $LINKAGE_PAIR restores the current psect when it is done.

    $LINKAGE_PAIR defines the symbol $LP. This symbol value is the
    address within the linkage section of the linkage pair for the
    specified routine to call.