add back NSIG, removed from powerpc in last commit, but for all archs

unlike the previous definition, NSIG/_NSIG is supposed to be one more
than the highest signal number. adding this will allow simplifying
libc-internal code that makes signal-related syscalls, which can be
done as a later step. some apps might use it too; while this usage is
questionable, it's at least not insane.

include/signal.h

@@ -217,6 +217,7 @@ void (*bsd_signal(int, void (*)(int)))(int);
 int sigisemptyset(const sigset_t *);
 #define SA_NOMASK SA_NODEFER
 #define SA_ONESHOT SA_RESETHAND
+#define NSIG _NSIG
 #endif
 
 #include <bits/signal.h>