arch-riscv: Allow to load raw binary image for riscv

[shinezyy]

• When loading raw binary image: --raw-bbl --kernel hello.bin
• When loading elf image: --kernel hello.elf
• The implementation is a little bit hacky: the addr of the _resetVect is manually injected into the RawImage object
• Decouple difftest from cpt mode by move pmem pointer update from PhysicalMemory::unserializeStoreFrom to PhysicalMemory::createBackingStore

Change-Id: I684d6eb1a11fd7ac332f4dc6f1d62e180d552d17