VxWorks timebase still registers signal with RTOS when sync function is not NULL

[jphickey]

Describe the bug
When calling OS_TimeBaseCreate with the 3rd argument (external_sync) non-NULL, it is supposed to use the external sync function rather than an register an RTOS timer to implement the tick. However, the VxWorks implementation still tries to register the RTOS timer, which has a signal number of 0, so it fails in timer_create().

To Reproduce
Run timebase-api-test on VxWorks, observe failure:

[ PASS] 01.012 time-base-api-test.c:116 - OS_TimeBaseCreate(&time_base_id2, "TimeBaseB", NULL) (0) == OS_SUCCESS (0)
OS_VxWorks_RegisterTimer():248:timer_create() failed: errno=22
OS_TimeBaseCreate_Impl():507:Error during timer registration
[ FAIL] 01.013 time-base-api-test.c:117 - OS_TimeBaseCreate(&time_base_id, "TimeBaseC", UT_TimerSync) (-32) == OS_SUCCESS (0)

Expected behavior
Test should pass

System observed on:
MCP750, VxWorks 6.9

Additional context
VxWorks just needs to skip timer_create() and other RTOS timer configs when assigned_signal is 0.

Reporter Info
Joseph Hickey, Vantage Systems, Inc.