Add remote gdb interfaces

The interfaces of remote gdb is in src/cpu/risc/cpu.hh. 
The last eigth functions.
Three for reading register x,y,g.
Three for writing register x,y,g.
One for reading pc.
One for writing pc.
When you want to use them, just using "context->getCpuPtr()->FUNCTION_NAME" in remote_gdb.cc.