linux-user/arm: Mark the commpage executable

We're about to start validating PAGE_EXEC, which means
that we've got to mark the commpage executable.  We had
been placing the commpage outside of reserved_va, which
was incorrect and lead to an abort.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>