Add _SC_NSIG.

https://pubs.opengroup.org/onlinepubs/9799919799.2024edition/functions/sysconf.html (2024) says "Austin Group Defect 741 is applied, adding _SC_NSIG".

Change-Id: Ic8372105bac153403ab41369805c46322c25013e
diff --git a/libc/include/bits/sysconf.h b/libc/include/bits/sysconf.h
index ecf26ba..303f7c6 100644
--- a/libc/include/bits/sysconf.h
+++ b/libc/include/bits/sysconf.h
@@ -328,6 +328,8 @@
 #define _SC_LEVEL4_CACHE_ASSOC 0x009c
 /** sysconf() query for the L4 cache line size. Not available on all architectures. */
 #define _SC_LEVEL4_CACHE_LINESIZE 0x009d
+/** sysconf() query equivalent to NSIG. Available from API level 37. */
+#define _SC_NSIG 0x009e
 
 __BEGIN_DECLS