Add configUSE_TASK_FPU_SUPPORT to AARCH64 port (#1048)

Add configUSE_TASK_FPU_SUPPORT to AARCH64 port (#1048)

* Add configUSE_TASK_FPU_SUPPORT to AARCH64 port

NEON SIMD is required by standard AARCH64 and its registers are
frequently utilized by standard functions such as memcpy().
This means that even simple tasks that do not use any floating point
arithmetics may still alter the contents of the FPU registers.

For this reason it makes sense to add support for
configUSE_TASK_FPU_SUPPORT to be able to enforce FPU register saving and
restoring globally.

The implementation was largely adopted from the ARM_CA9 port. However,
the FPU registers must be placed on the stack before the critical
nesting count to match the AARCH64 portASM.S.