sys/prctl.h: add PR_SPEC_DISABLE_NOEXEC from linux v5.1

allows specifying that the speculative store bypass disable bit should
be cleared on exec. see

  linux commit 71368af9027f18fe5d1c6f372cfdff7e4bde8b48
  x86/speculation: Add PR_SPEC_DISABLE_NOEXEC

include/sys/prctl.h

@@ -145,6 +145,7 @@ struct prctl_mm_map {
 #define PR_SPEC_ENABLE (1UL << 1)
 #define PR_SPEC_DISABLE (1UL << 2)
 #define PR_SPEC_FORCE_DISABLE (1UL << 3)
+#define PR_SPEC_DISABLE_NOEXEC (1UL << 4)
 
 #define PR_PAC_RESET_KEYS       54
 #define PR_PAC_APIAKEY (1UL << 0)