sim-se: Fix tgkill logic bug in handling signal argument (#286)

The syscall emulation of tgkill contained a simple logic bug (a `||`
instead of a `&&`), causing the signal argument to always be considered
invalid. This patch fixes the bug by simply changing the `||` to a `&&`.

GitHub issue: https://github.com/gem5/gem5/issues/284

src/sim/syscall_emul.hh

@@ -2409,7 +2409,7 @@ tgkillFunc(SyscallDesc *desc, ThreadContext *tc, int tgid, int tid, int sig)
         }
     }
 
-    if (sig != 0 || sig != OS::TGT_SIGABRT)
+    if (sig != 0 && sig != OS::TGT_SIGABRT)
         return -EINVAL;
 
     if (tgt_proc == nullptr)