FreeRTOS-Kernel Improve multiple accessing pxCurrentTCB in a function

Description

Address Richard's feedback to update the usage of pxCurrentTCB in a function. Since pxCurrentTCB is a volatile variable, accessing it multiple times in a function can be slow if this pointer is not changed.

In this PR:

Use a const pointer variable to access pxCurrentTCB in a function to prevent multiple accessing a volatile variable.
The prvGetCurrentTCB() and prvGetCurrentTCBUnsafe() functions are provided. prvGetCurrentTCBUnsafe() can improve performance in SMP environments as it does not disable/enable interrupts. However, it should only be called in contexts where race conditions cannot occur.

Test Steps

The performance gain is evaluated by comparing main branch with this PR with a hardware cycle counter.

We use the following example to illustrate the performance gain on ulTaskGenericNotifyTake.

static void PerfTaskNotifyHigh( void * pvParameter )
{
    ( void ) pvParameter;

    ulTaskNotifyTake( pdTRUE, portMAX_DELAY );

    perfTestEndCycles = HardwareCounter_Read();
    HardwareCounter_Stop();

    vTaskDelete( NULL );
}

uint32_t PerfTest_xTaskNotifyGive_UnblockHighTask( void )
{
    TaskHandle_t notifyTaskHandle;

    xTaskCreate( PerfTaskNotifyHigh,
                 "PerfTaskNotifyHigh",
                 PERFTEST_TASK_STACK_SIZE,
                 &pTaskPerfTest,
                 PERFTEST_TASK_PRIORITY_HIGH,
                 &notifyTaskHandle );

    HardwareCounter_Start();
    perfTestStartCycles = HardwareCounter_Read();

    xTaskNotifyGive( notifyTaskHandle );

    /* The high priority task delete itself. Wait idle task to free the resource. */
    vTaskDelay( 1 );

    return( perfTestEndCycles - perfTestStartCycles );
}

In the example above, bottom half of the ulTaskGenericNotifyTake are evaluated with compile option ( -O3 ).

Cycle counts difference	main branch	This PR	Performance gain
PerfTest_xTaskNotifyGive_UnblockHighTask	485	465	20

Checklist:

[x] I have tested my changes. No regression in existing tests.
[x] I have modified and/or added unit-tests to cover the code changes in this Pull Request. - https://github.com/FreeRTOS/FreeRTOS/pull/1235