cmake: x86: Add wrapper targets for custom commands

In general, to have correct dependencies, one must not only depend on
files, but also a wrapper target for the file. This is done for some
of the files in arch/x86/CMakeLists.txt, but not all.

To be consistent with how dependency management is done we add wrapper
targets and add dependencies to them.

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