linux-user/FLAT: fix auto-stack sizing

linux-user/FLAT: fix auto-stack sizing

The current auto-stack sizing works like it does on a NOMMU system; the
problem is that this only works if the envp/argv arrays are fairly slim.
On a desktop system, this is rarely the case, and can easily blow past
the stack and into data/text regions as the default stack for FLAT progs
is a mere 4KiB.  So rather than rely on the NOMMU calculation (which is
only there because NOMMU can't easily allocate gobs of contiguous mem),
calc the full space actually needed and let the MMU host make space.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Riku Voipio <riku.voipio@nokia.com>