Extract NEC Vr54xx helpers from op_helper.c to a new file:
'vr54xx_helper.c'.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201120210844.2625602-14-f4bug@amsat.org>