linux-user: fix multi-threaded /proc/self/maps

When reading our faked /proc/self/maps from a secondary thread,
we get an invalid stack entry. This is because ts->stack_base is not
initialized in non-primary threads.

However, ts->info is, and the stack layout information we're looking
for is there too. So let's use that one instead!

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
