linux-user: Allow TARGET_PAGE_BITS_VARY

If set, match the host and guest page sizes.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Helge Deller <deller@gmx.de>
Message-Id: <20240102015808.132373-30-richard.henderson@linaro.org>