linux-user: Adjust SVr4 NULL page mapping

linux-user: Adjust SVr4 NULL page mapping

Use TARGET_PAGE_SIZE and MAP_FIXED_NOREPLACE.

We really should be attending to this earlier during
probe_guest_base, as well as better detection and
emulation of various Linux personalities.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Acked-by: Helge Deller <deller@gmx.de>
Message-Id: <20240102015808.132373-3-richard.henderson@linaro.org>