linux-user: init_guest_space: Try to make ARM space+commpage continuous

At a fixed distance after the usable memory that init_guest_space maps, for
32-bit ARM targets we also need to map a commpage.  The normal
init_guest_space logic doesn't keep this in mind when searching for an
address range.

If !host_start, then try to find a big continuous segment where we can put
both the usable memory and the commpage; we then munmap that segment and
set current_start to that address; and let the normal code mmap the usable
memory and the commpage separately.  That is: if we don't have hint of
where to start looking for memory, come up with one that is better than
NULL.  Depending on host_size and guest_start, there may or may not be a
gap between the usable memory and the commpage, so this is slightly more
restrictive than it needs to be; but it's only a hint, so that's OK.

We only do that for !host start, because if host_start, then either:
 - we got an address passed in with -B, in which case we don't want to
   interfere with what the user said;
 - or host_start is based off of the ELF image's loaddr.  The check "if
   (host_start && real_start != current_start)" suggests that we really
   want lowest available address that is >= loaddr.  I don't know why that
   is, but I'm trusting that Paul Brook knew what he was doing when he
   wrote the original version of that check in
   c581deda way back in 2010.

Signed-off-by: Luke Shumaker <lukeshu@parabola.nu>
Message-Id: <20171228180814.9749-11-lukeshu@lukeshu.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>