tcg: Introduce target-specific page data for user-only

This data can be allocated by page_alloc_target_data() and
released by page_set_flags(start, end, prot | PAGE_RESET).

This data will be used to hold tag memory for AArch64 MTE.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210212184902.1251044-2-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>