x86: enable 4G high memory support for generic (32bit) subtarget

This is required to fully support a number of 32bit x86 systems equipped
with more than one GB ram, e.g. certain Atom CPU machines.

Ref: https://forum.lede-project.org/t/17-01-1-x86-doesnt-detect-all-of-the-ram/3295



Signed-off-by: Jo-Philipp Wich <jo@mein.io>