scripts: size_report: Use default encoding to process filenames

The linker generates an .elf file wth encoded strings containing
filenames. If those filenames contain non-ASCII characters we need to
know which encoding was used by the linker to store those filenames. Use
the system's preferred encoding for that purpose.

Fixes #55148.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>