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/linux/epoll.c

@@ -1,4 +1,5 @@
 #include <sys/epoll.h>
+#include <signal.h>
 #include "syscall.h"
 
 int epoll_create(int size)
@@ -18,7 +19,7 @@ int epoll_ctl(int fd, int op, int fd2, struct epoll_event *ev)
 
 int epoll_pwait(int fd, struct epoll_event *ev, int cnt, int to, const sigset_t *sigs)
 {
-	return syscall(SYS_epoll_pwait, fd, ev, cnt, to, sigs, __SYSCALL_SSLEN);
+	return syscall(SYS_epoll_pwait, fd, ev, cnt, to, sigs, _NSIG/8);
 }
 
 int epoll_wait(int fd, struct epoll_event *ev, int cnt, int to)

src/linux/ppoll.c

@@ -1,9 +1,10 @@
 #define _GNU_SOURCE
 #include <poll.h>
+#include <signal.h>
 #include "syscall.h"
 
 int ppoll(struct pollfd *fds, nfds_t n, const struct timespec *to, const sigset_t *mask)
 {
 	return syscall_cp(SYS_ppoll, fds, n,
-		to ? (struct timespec []){*to} : 0, mask, __SYSCALL_SSLEN);
+		to ? (struct timespec []){*to} : 0, mask, _NSIG/8);
 }

src/linux/signalfd.c

@@ -1,7 +1,8 @@
 #include <sys/signalfd.h>
+#include <signal.h>
 #include "syscall.h"
 
 int signalfd(int fd, const sigset_t *sigs, int flags)
 {
-	return syscall(SYS_signalfd, fd, sigs, __SYSCALL_SSLEN);
+	return syscall(SYS_signalfd, fd, sigs, _NSIG/8);
 }