alpha: Support cpu-specific palcode instructions

In qemu, our emulated firmware uses the EV6 palcode instructions: hw_ld, hw_st, hw_ret, hw_mfpr, hw_mtpr.

See Alpha 21264 Microprocessor Data Sheet section 5.4 Opcodes Reserved for PALcode. Alternately, binutils alpha-opc.c which has the palcode instructions for EV5, EV5, and EV6 all in one place.