cmake: new global property for generated files

Introduce a new global property for source files that are generated from
the initial Zephyr link (app_smem_unaligned_prebuilt). This source list
is used for files which will introduce address shifts into the final
binary, which need to be present in `zephyr_prebuilt.elf` for
`CONFIG_USERSPACE` scripts to correctly generate `zephyr.elf`.

This resolves #38836.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>