add AT_MINSIGSTKSZ to elf.h from linux v4.18

new in linux commit 94b07c1f8c39c6d839df35fa28ffd1785d385897

currently only supported on aarch64

include/elf.h

@@ -1035,6 +1035,7 @@ typedef struct {
 #define AT_L3_CACHESIZE		46
 #define AT_L3_CACHEGEOMETRY	47
 
+#define AT_MINSIGSTKSZ		51
 
 
 typedef struct {