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/x86_64/mod.rs

@@ -53,6 +53,8 @@ const KVM_FEATURE_ASYNC_PF_VMEXIT_BIT: u8 = 10;
 #[cfg(feature = "tdx")]
 const KVM_FEATURE_STEAL_TIME_BIT: u8 = 5;
 
+pub const _NSIG: i32 = 65;
+
 #[derive(Debug, Copy, Clone)]
 /// Specifies the entry point address where the guest must start
 /// executing code, as well as which of the supported boot protocols