Compiler Warnings in tasks.c

A very minor point. When building FreeRTOS 4.7.1 on my 16bit target, I get a couple of compiler warning in tasks.c. These are where in the xTaskGenericCreate() function thereare  assertions checking the stack alignment at lines 534 (or 541 depending which way your stack goes) and 568. The code uses ( unsigned long ) as a cast instead of ( portPOINTER_SIZE_TYPE ).