remove __SYSCALL_SSLEN arch macro in favor of using public _NSIG

the issue at hand is that many syscalls require as an argument the
kernel-ABI size of sigset_t, intended to allow the kernel to switch to
a larger sigset_t in the future. previously, each arch was defining
this size in syscall_arch.h, which was redundant with the definition
of _NSIG in bits/signal.h. as it's used in some not-quite-portable
application code as well, _NSIG is much more likely to be recognized
and understood immediately by someone reading the code, and it's also
shorter and less cluttered.

note that _NSIG is actually 65/129, not 64/128, but the division takes
care of throwing away the off-by-one part.

src/time/timer_create.c

@@ -53,7 +53,7 @@ static void install_handler()
 	};
 	__libc_sigaction(SIGTIMER, &sa, 0);
 	__syscall(SYS_rt_sigprocmask, SIG_UNBLOCK,
-		SIGTIMER_SET, 0, __SYSCALL_SSLEN);
+		SIGTIMER_SET, 0, _NSIG/8);
 }
 
 static void *start(void *arg)