common-user: Move mmap_min_addr from linux-user

Introduce user/mmap-min-addr.h.  Initialize the variable
from a constructor instead of main.

Reviewed-by: Helge Deller <deller@gmx.de>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>