cmake: move configure_linker_script to ld/target.cmake

While configure_linker_script() may be useful for other linkers, it
currently only aimed at GNU ld. To really be useful among different
linkers, we would need to abstract its usage of the C preprocessor.
We can do this later, if needed.

No functional change expected.

Signed-off-by: Mark Ruvald Pedersen <mped@oticon.com>