Add support for the configUSE_TASK_FPU_SUPPORT constant in the GCC/ARM_CR5 port ...

Add support for the configUSE_TASK_FPU_SUPPORT constant in the GCC/ARM_CR5 port (#584)

* Add support for the configUSE_TASK_FPU_SUPPORT in the GCC/ARM_CR5 port

This is done almost identically as in the GCC/ARM_CA9 port

* Adjust task stack initialitation of the GCC/ARM_CR5 port

Ensure that the task stack initialization is done correctly for the
different options of configUSE_TASK_FPU_SUPPORT.

This is very similar to the GCC/ARM_CA9 port. The only meaningful
difference is, that the FPU of the Cortex-R5 has just sixteen 64-bit
floating point registers as it implements the VFPv3-D16 architecture.
You may also refer to the ARM documentation

* Add support for FPU safe interrupts to the GCC/ARM_CR5 port

Similar to GCC/ARM_CA9 port

* Clarify comment about the size of the FPU registers of Cortex R5