ARC: linker: merge GNU and MWDT linker scripts

As discussed in #22668, there is additional risk ascociated with
splitting linker files, as one may update one script and not be
aware of the other. Especially related to updating GNU ld, and
not mwdt could break code for mwdt unnoticed, as mwdt is not
part of CI.

Let's create a single entry point for linker template.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>