riscv: support extension from mode Hu (16 bit) to machine size
riscv: support extension from mode Hu (16 bit) to machine size

libfirm/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,libfirm/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm