tcg: Drop gen_io_end()

tcg: Drop gen_io_end()

Now we have removed all the uses of gen_io_end() from target frontends,
the only callsite is inside gen_tb_start(). Inline the code there,
and remove the reference to it from the documentation.

While we are inlining the code, switch it to use tcg_constant_i32()
so we don't have to manually create and destroy a TCG temporary.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210724134902.7785-3-peter.maydell@linaro.org>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>