Fix rts allocateExec() on NetBSD

Similar to SELinux, NetBSD "PaX mprotect" prohibits marking a page
mapping both writable and executable at the same time. Use libffi
which knows how to work around it.

rts/sm/Storage.c

@@ -1595,11 +1595,12 @@ void flushExec (W_ len, AdjustorExecutable exec_addr)
 #endif
 }
 
-#if defined(linux_HOST_OS)
+#if defined(linux_HOST_OS) || defined(netbsd_HOST_OS)
 
 // On Linux we need to use libffi for allocating executable memory,
 // because it knows how to work around the restrictions put in place
-// by SELinux.
+// by SELinux. The same goes for NetBSD where it is prohibited to
+// mark a page mapping both writable and executable at the same time.
 
 AdjustorWritable allocateExec (W_ bytes, AdjustorExecutable *exec_ret)
 {