fix building when __NR_sigalstack does not exist

libc/sysdeps/linux/common/sigaltstack.c

@@ -9,5 +9,8 @@
 
 #include <sys/syscall.h>
 #include <signal.h>
+
+#ifdef __NR_sigaltstack
 _syscall2(int, sigaltstack, const struct sigaltstack *, ss,
 		  struct sigaltstack *, oss);
+#endif