Implement pthread_cancel.

core/sys/unix/pthread_openbsd.odin

@@ -46,6 +46,11 @@ sched_param :: struct {
 
 sem_t :: distinct rawptr
 
+PTHREAD_CANCEL_ENABLE       :: 0
+PTHREAD_CANCEL_DISABLE      :: 1
+PTHREAD_CANCEL_DEFERRED     :: 0
+PTHREAD_CANCEL_ASYNCHRONOUS :: 1
+
 foreign import libc "system:c"
 
 @(default_calling_convention="c")
@@ -62,4 +67,8 @@ foreign libc {
 	// NOTE: unclear whether pthread_yield is well-supported on Linux systems,
 	// see https://linux.die.net/man/3/pthread_yield
 	pthread_yield :: proc() ---
-}
+
+	pthread_setcancelstate :: proc (state: c.int, old_state: ^c.int) -> c.int ---
+	pthread_setcanceltype  :: proc (type:  c.int, old_type:  ^c.int) -> c.int ---
+	pthread_cancel         :: proc (thread: pthread_t) -> c.int ---
+}