testsuite: disable T13615 on non-smp targets
testsuite: disable T13615 on non-smp targets

T13615 needs multicore support from RTS:
    T13615: unknown RTS option: -N15

Signed-off-by: Sergei Trofimovich <110fdadc76b6bce16977f568bdb4723eaed6fb3f@gentoo.org>

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc