fix(sync/wait_condition): deadlock in WaitCondition::wait

Instead of using a simple switch to determine if preemption is enabled
(`is_preemptable: bool`), a counter is used instead. This handles the
case where a function holding a `PreemptGuard` calls another function
that creates a new `PreemptGuard`.

Signed-off-by: Anhad Singh <andypython@protonmail.com>

src/context/switch.rs

@@ -165,7 +165,7 @@ pub fn switch(token: &mut CleanLockToken) -> SwitchResult {
         // We are careful not to lock this context twice
         let prev_context_guard = unsafe { prev_context_lock.write_arc() };
 
-        if !prev_context_guard.is_preemptable {
+        if !prev_context_guard.is_preemptable() {
             return SwitchResult::AllContextsIdle;
         }