user: Extract common MMAP API to 'user/mmap.h'

Keep common MMAP-related declarations in a single place.

Note, this disable ThreadSafetyAnalysis on Linux for:
- mmap_fork_start()
- mmap_fork_end().

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250308122842.76377-4-philmd@linaro.org>
3 files changed