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/internal/pthread_impl.h

@@ -41,7 +41,7 @@ struct pthread {
 	int killlock[2];
 	int exitlock[2];
 	int startlock[2];
-	unsigned long sigmask[__SYSCALL_SSLEN/sizeof(long)];
+	unsigned long sigmask[_NSIG/8/sizeof(long)];
 };
 
 struct __timer {
@@ -89,10 +89,10 @@ struct __timer {
 
 #define SIGALL_SET ((sigset_t *)(const unsigned long long [2]){ -1,-1 })
 #define SIGPT_SET \
-	((sigset_t *)(const unsigned long [__SYSCALL_SSLEN/sizeof(long)]){ \
+	((sigset_t *)(const unsigned long [_NSIG/8/sizeof(long)]){ \
 	[sizeof(long)==4] = 3UL<<(32*(sizeof(long)>4)) })
 #define SIGTIMER_SET \
-	((sigset_t *)(const unsigned long [__SYSCALL_SSLEN/sizeof(long)]){ \
+	((sigset_t *)(const unsigned long [_NSIG/8/sizeof(long)]){ \
 	 0x80000000 })
 
 pthread_t __pthread_self_init(void);