target/riscv: convert to DisasContextBase

target/riscv: convert to DisasContextBase

Notes:

- Did not convert {num,max}_insns, since the corresponding code
  will go away in the next patch.

- ctx->pc becomes ctx->base.pc_next, and ctx->next_pc becomes
  ctx->pc_succ_insn.

While at it, convert the remaining tb->cflags readers to tb_cflags().

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: Michael Clark <mjc@sifive.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>