kernel: Add Z_IS_TIMEOUT_RELATIVE() macro

Introduces the Z_IS_TIMEOUT_RELATIVE() macro to help ensure that
checking for relative/absolute timeouts is consistent. Using this
macro also helps ensure that we get the correct behavior when using
32-bit timeouts (CONFIG_TIMEOUT_64BIT=n).

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>