exec.c: Remove static allocation of sub_section of sub_page

exec.c: Remove static allocation of sub_section of sub_page

Allocate sub_section dynamically. Remove dependency
on TARGET_PAGE_SIZE to make run-time page size detection
for arm platforms.

Signed-off-by: Vijaya Kumar K <vijayak@cavium.com>
Message-id: 1465808915-4887-3-git-send-email-vijayak@caviumnetworks.com
[PMM: use flexible array member rather than separate malloc
 so we don't need an extra pointer deref when using it]
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>