POSIX/t/sigaction.t: Skip failing EBCDIC test

POSIX/t/sigaction.t: Skip failing EBCDIC test

We don't know why this is faiing, but it also fails on another IBM
product, AIX 4.2.  Skip for now.  The code that is failing is this:
    sub hiphup {
        is($_[1]->{signo}, SIGHUP, "SA_SIGINFO got right signal");
    }
    my $act = POSIX::SigAction->new('hiphup', 0, SA_SIGINFO);
    sigaction(SIGHUP, $act);
    kill 'HUP', $$;
and it leads to this:
CEE3206S The system detected a specification exception (System Completion Code=0C6).
         From entry point Perl_apply at compile unit offset +00001460 at entry offset +00001460 at address 1F9B4A50.
ext/POSIX/t/sigaction ......................................... FAILED--expected 33 tests, saw 30

A stack trace with some analysis is in email <5490A778.30308@lerctr.org>