cmake: x86: Refactor build scripts

The same pattern is used five times In the x86 build scripts and the
same code has been copied and modified the same amount of times. This
has resulted in a system that is difficult to make changes to.

To enforce consistency and improve maintainability we refactor the
code into a function.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>