x86-64: Move LP_SIZE definition to its own header

x86-64: Move LP_SIZE definition to its own header

This way, we can define the pointer guard macros without including
<sysdep.h> on x86-64.  Other architectures will not have such an
inclusion dependency, and the implied header file inclusion would
create a porting hazard.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>