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/select/pselect.c

@@ -1,10 +1,11 @@
 #include <sys/select.h>
+#include <signal.h>
 #include "syscall.h"
 #include "libc.h"
 
 int pselect(int n, fd_set *restrict rfds, fd_set *restrict wfds, fd_set *restrict efds, const struct timespec *restrict ts, const sigset_t *restrict mask)
 {
-	long data[2] = { (long)mask, __SYSCALL_SSLEN };
+	long data[2] = { (long)mask, _NSIG/8 };
 	struct timespec ts_tmp;
 	if (ts) ts_tmp = *ts;
 	return syscall_cp(SYS_pselect6, n, rfds, wfds, efds, ts ? &ts_tmp : 0, data);