[ARM] arm64: Minor improvments to VFP context switching

This patch does following improvements to VCPU context
switching in arm64:
1. We only need array of 64x 64bit variable to store all
32x 128bit VFP data registers
2. We don't need to check everytime whether underlying
host HW supports FPU or not.

Signed-off-by: Anup Patel <anup@brainfault.org>