cmake: linker generator replace % with @ for symbol referencing

Fixes: #41435

The use of %<symbol>% works well in Linux and MacOS but when passed
to the linker script generator in Windows the %<symbol>% is processed
by the windows shell causing the value to disappear and thus an empty
value inside the generated ld linker script or scatter file.

This is fixed by using the @ character instead of the % character.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>