Added workaround for armc assumptions on integer underflow

With ARM Compiler 5.06u3, when the equeue_tickdiff function is
inlined, the compiler treats the unsigned integer subtraction
as undefined behaviour and assumes falsely the comparisons
could never be true.

The workaround is to an explicit cast, which politely reminds the
compiler to emit the correct comparison.

equeue.c

@@ -13,7 +13,7 @@
 // calculate the relative-difference between absolute times while
 // correctly handling overflow conditions
 static inline int equeue_tickdiff(unsigned a, unsigned b) {
-    return (int)(a - b);
+    return (int)(unsigned)(a - b);
 }
 
 // calculate the relative-difference between absolute times, but