sys: clock: re-apply change to Z_IS_TIMEOUT_RELATIVE

A mid-air collision between two different changes to sys_clock.h
resulted in a test failure.

```shell
tests/kernel/timer/timer_api/src/main.c:763:
  timer_api_test_timeout_abs: Z_IS_TIMEOUT_RELATIVE(t2) is false
```

Re-apply the change that was added in

9075d53

to keep tests running smoothly.

Signed-off-by: Chris Friedt <[email protected]>

Author: cfriedt

include/zephyr/sys/clock.h

@@ -151,7 +151,14 @@ typedef struct {
 
 /* Test for relative timeout */
 #if CONFIG_TIMEOUT_64BIT
-#define Z_IS_TIMEOUT_RELATIVE(timeout) (Z_TICK_ABS((timeout).ticks) < 0)
+/* Positive values are relative/delta timeouts and negative values are absolute
+ * timeouts, except -1 which is reserved for K_TIMEOUT_FOREVER. 0 is K_NO_WAIT,
+ * which is historically considered a relative timeout.
+ * K_TIMEOUT_FOREVER is not considered a relative timeout and neither is it
+ * considerd an absolute timeouts (so !Z_IS_TIMEOUT_RELATIVE() does not
+ * necessarily mean it is an absolute timeout if ticks == -1);
+ */
+#define Z_IS_TIMEOUT_RELATIVE(timeout) (((timeout).ticks) >= 0)
 #else
 #define Z_IS_TIMEOUT_RELATIVE(timeout) true
 #endif