scripts: gen_relocate_app: Use linker provided size variable.

This seems to solve the issue with GH-12033. There seems to be
some compiler optimization that was causing this issue. This
occurs only when the build is re-run by invoking cmake without
clearing the previous build.

Fixes:GH-12033

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>