linux-user/nios2: Only initialize SP and PC in target_cpu_copy_regs

Drop the set of estatus in init_thread; it was clearly intended
to be setting the value of CR_STATUS for the application, but we
never actually performed that copy.  However, the proper value is
set in nios2_cpu_reset so we don't need to do anything here.

We only initialize SP and EA in init_thread, there's no value in
copying other uninitialized data into ENV.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220421151735.31996-21-richard.henderson@linaro.org>