ld/arc: Change default linker emulation for nps400

If we are configuring for an arc/linux target, and --with-cpu=nps400 is
used at configure time then change the default linker emulation to the
nps specific version.  All of the alternative linker emulations are
still available using the -mNAME option for ld.

ld/ChangeLog:

	* configure.tgt (arc*-*-linux*): Change the default linker
	emulation based on --with-cpu selection.
	* NEWS: Mention new configuration option.