There are two places that need -lm (test app and librte_sched) and
exactly one that needs -lrt (librte_sched). Add the relevant
DT_NEEDED entries to both, and eliminate the bogus discrepancy
between Linux and BSD EXECENV_LDLIBS wrt these libs.
Signed-off-by: Panu Matilainen <pmatilai@redhat.com>