target/nios2: Split out helper for eret instruction

target/nios2: Split out helper for eret instruction

The implementation of eret will become much more complex
with the introduction of shadow registers.

[rth: Split out of a larger patch for shadow register sets.
      Directly exit to the cpu loop from the helper.]

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Amir Gonnen <amir.gonnen@neuroblade.ai>
Message-Id: <20220303153906.2024748-3-amir.gonnen@neuroblade.ai>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220421151735.31996-18-richard.henderson@linaro.org>