linux-user: fix mips 32-on-64 prealloc case

MIPS only supports 31 bits of virtual address space for user space, so let's
make sure we stay within that limit with our preallocated memory block.

This fixes the MIPS user space targets when executed without command line
option.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
