signal/alpha: Replace TRAP_FIXME with TRAP_UNK

signal/alpha:  Replace TRAP_FIXME with TRAP_UNK

Using an si_code of 0 that aliases with SI_USER is clearly the wrong
thing to do, and causes problems in interesting ways.

For it really is not clear to me if using TRAP_UNK bugcheck or
the default case of gentrap is really the best way to handle
things.  There is certainly enough information that that a more
specific si_code could potentially be used.  That said TRAP_UNK
is definitely an improvement over 0 as it removes the ambiguiuty
of what si_code of 0 with SIGTRAP means on alpha.

Recent history suggests no actually cares about crazy corner cases of
the kernel behavior like this so I don't expect any regressions from
changing this.  However if something does happen this change is easy
to revert.

Cc: Helge Deller <deller@gmx.de>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: linux-alpha@vger.kernel.org
Fixes: 0a635c7a84cf ("Fill in siginfo_t.")
History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>