risugen: Handle reg+imm load/stores
risugen: Handle reg+imm load/stores

Add the support code for setup and teardown of the memory
address for loads and stores with reg+imm addressing mode.
vivier/risu,stsquad/risu,hw-claudio/risu-aarch64,stsquad/risu,stsquad/risu,hw-claudio/risu-aarch64,vivier/risu