exchangers: Add accessor function.

src/xt_exchanger_irecv_isend.c

src/xt_exchanger_irecv_isend_ddt_packed.c

src/xt_exchanger_irecv_isend_packed.c

src/xt_exchanger_irecv_send.c

src/xt_exchanger_simple_base.c

src/xt_exchanger_simple_base.h

src/xt_exchanger_simple_base_internal.h