perf trace: Decode architecture-specific signal numbers

SIGSTKFLT is not defined on alpha, mips or sparc.

SIGEMT and SIGSWI are defined on some architectures and should be
decoded here if so.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Fixes: 8bad5b0a

 ('perf trace: Beautify signal number arg in several syscalls')
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/1391648441.3003.101.camel@deadeye.wl.decadent.org.uk
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>