Merge init_thread and target_cpu_copy_regs.
There's no point going through a target_pt_regs intermediate.
Note that init_thread had set crmd in target_pt_regs, but
target_cpu_copy_regs did not copy to env. This turns out to be
ok because loongarch_cpu_reset_hold initializes CRMD properly.
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>