drop use of execstack on x86_64
    
    BUILDSTDERR: execstack: dso.c:877: reopen_dso: Assertion `dso->shdr[j].sh_size == 0' failed.
    BUILDSTDERR: /var/tmp/rpm-tmp.3AqnlA: line 39:  5224 Aborted                 (core dumped) execstack -s /builddir/build/BUILDROOT/hugs98-2006.09-32.fc30.x86_64/usr/bin/{hugs,runhugs,ffihugs}