RISC-V: Use riscv prefix consistently on cpu helpers

RISC-V: Use riscv prefix consistently on cpu helpers

* Add riscv prefix to raise_exception function
* Add riscv prefix to CSR read/write functions
* Add riscv prefix to signal handler function
* Add riscv prefix to get fflags function
* Remove redundant declaration of riscv_cpu_init
  and rename cpu_riscv_init to riscv_cpu_init
* rename riscv_set_mode to riscv_cpu_set_mode

Signed-off-by: Michael Clark <[email protected]>
Signed-off-by: Alistair Francis <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: Palmer Dabbelt <[email protected]>