sys/prctl.h: add new PR_SVE_* macros from linux v4.15

PR_SVE_SET_VL and PR_SVE_GET_VL controls are new in linux commit
2d2123bc7c7f843aa9db87720de159a049839862
related PR_SVE_* macros were added in
7582e22038a266444eb87bc07c372592ad647439

include/sys/prctl.h

@@ -130,6 +130,12 @@ struct prctl_mm_map {
 #define PR_CAP_AMBIENT_LOWER    3
 #define PR_CAP_AMBIENT_CLEAR_ALL 4
 
+#define PR_SVE_SET_VL           50
+#define PR_SVE_SET_VL_ONEXEC (1 << 18)
+#define PR_SVE_GET_VL           51
+#define PR_SVE_VL_LEN_MASK 0xffff
+#define PR_SVE_VL_INHERIT (1 << 17)
+
 int prctl (int, ...);
 
 #ifdef __cplusplus