vmm: fall back if CLONE_CLEAR_SIGHAND unsupported

This will allow the SIGWINCH listener to run on kernels older than
5.5, although on those kernels it will have to make 64 syscalls to
reset all the signal handlers.

Signed-off-by: Alyssa Ross <hi@alyssa.is>

arch/src/aarch64/mod.rs

@@ -21,6 +21,8 @@ use std::fmt::Debug;
 use std::sync::{Arc, Mutex};
 use vm_memory::{Address, GuestAddress, GuestMemory, GuestMemoryAtomic, GuestUsize};
 
+pub const _NSIG: i32 = 65;
+
 /// Errors thrown while configuring aarch64 system.
 #[derive(Debug)]
 pub enum Error {