arm: armv8 boot options to enable v8

arm: armv8 boot options to enable v8

Modifies FSConfig.py to enable ARMv8 compatibility.
To boot gem5 with ARMv8:
   Download the v8 kernel, .dtb file, and root FS from: http://gem5.org/Download
   Download the ARMv8 toolchain, and add the bin dir to your path:
       http://www.linaro.org/engineering/engineering-projects/armv8
   Build gem5 for ARM
   Build the v8 bootloader (in gem5/system/arm/aarch64_bootloader)
   Make script in gem5/system/arm/aarch64_bootloader will require v8 toolchain,
   drop the produced boot_emm.arm64 in $(M5_PATH)/binaries/
Run:
   $ build/ARM/gem5.fast configs/example/fs.py --machine-type=VExpress_EMM64 \
     --kernel=/path/to/kernel/vmlinux-linaro-tracking \
     --dtb-filename=/path/to/dtb/rtsm_ve-aemv8a.dtb \
     --disk-image=/path/to/img/linaro-minimal-armv8.img