Revert "linux-user/arm: Take more care allocating commpage"

This reverts commit 4f5c67f8.

This exposes bugs in target_mmap et al with respect to overflow
with the final page of the guest address space.  To be fixed in
the next development cycle.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>