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/signal/raise.c

@@ -8,10 +8,10 @@ int raise(int sig)
 {
 	int pid, tid, ret;
 	sigset_t set;
-	__syscall(SYS_rt_sigprocmask, SIG_BLOCK, SIGALL_SET, &set, __SYSCALL_SSLEN);
+	__syscall(SYS_rt_sigprocmask, SIG_BLOCK, SIGALL_SET, &set, _NSIG/8);
 	tid = syscall(SYS_gettid);
 	pid = syscall(SYS_getpid);
 	ret = syscall(SYS_tgkill, pid, tid, sig);
-	__syscall(SYS_rt_sigprocmask, SIG_SETMASK, &set, 0, __SYSCALL_SSLEN);
+	__syscall(SYS_rt_sigprocmask, SIG_SETMASK, &set, 0, _NSIG/8);
 	return ret;
 }

src/signal/siglongjmp.c

@@ -6,6 +6,6 @@
 _Noreturn void siglongjmp(sigjmp_buf buf, int ret)
 {
 	if (buf->__fl) __syscall(SYS_rt_sigprocmask, SIG_SETMASK,
-		buf->__ss, 0, __SYSCALL_SSLEN);
+		buf->__ss, 0, _NSIG/8);
 	longjmp(buf->__jb, ret);
 }

src/signal/sigpending.c

@@ -3,5 +3,5 @@
 
 int sigpending(sigset_t *set)
 {
-	return syscall(SYS_rt_sigpending, set, __SYSCALL_SSLEN);
+	return syscall(SYS_rt_sigpending, set, _NSIG/8);
 }

src/signal/sigsuspend.c

@@ -4,5 +4,5 @@
 
 int sigsuspend(const sigset_t *mask)
 {
-	return syscall_cp(SYS_rt_sigsuspend, mask, __SYSCALL_SSLEN);
+	return syscall_cp(SYS_rt_sigsuspend, mask, _NSIG/8);
 }

src/signal/sigtimedwait.c

@@ -7,7 +7,7 @@ int sigtimedwait(const sigset_t *restrict mask, siginfo_t *restrict si, const st
 {
 	int ret;
 	do ret = syscall_cp(SYS_rt_sigtimedwait, mask,
-		si, timeout, __SYSCALL_SSLEN);
+		si, timeout, _NSIG/8);
 	while (ret<0 && errno==EINTR);
 	return ret;
 }