kernel: add a compatibility commpage on x86_64.

* x86 uses a commpage with 32-bit addresses, incompatible with the one used for
x86_64. For this reason, a compatibility commpage is needed to support a 32-bit
userland on x86_64.
* define ADDRESS_TYPE as a macro for addr_t (default) or uint32 (for the 32-bit
commpage).
* team_create_thread_start_internal() will use clone_commpage_area() with
KERNEL_USER_DATA_BASE or clone_commpage_compat_area() with
KERNEL_USER32_DATA_BASE, to setup the correct commpage.
* real_time_clock (in compatibility mode) also updates the compatibility
commpage with real time data.

Change-Id: I61605077ce0beabab4439ef54edd1eae26f26fd2