BUILD: threads: Add __ha_cas_dw fallback for single threaded builds

__ha_cas_dw() is used in fd_rm_from_fd_list() and when built without
USE_THREADS=1 the linker fails to find __ha_cas_dw(). Add a definition
of __ha_cas_dw() for the #ifndef USE_THREADS case.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>

include/common/hathreads.h

@@ -153,6 +153,11 @@ static inline void __ha_barrier_full(void)
 {
 }
 
+static inline int __ha_cas_dw(void *target, void *compare, void *set)
+{
+	return HA_ATOMIC_CAS(target, compare, set);
+}
+
 static inline void thread_harmless_now()
 {
 }