target/riscv: Use both register name and ABI name

target/riscv: Use both register name and ABI name

Use both the generic register name and ABI name for the general purpose
registers and floating point registers.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>