linux-user/arm: Take more care allocating commpage

User setting of -R reserved_va can lead to an assertion
failure in page_set_flags.  Sanity check the value of
reserved_va and print an error message instead.  Do not
allocate a commpage at all for m-profile cpus.

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