ld -plugin options when plugins are disabled

ld -plugin options when plugins are disabled

This patch makes ld accept and ignore -plugin options when configured
with --disable-plugins.  The idea is to allow using a linker built
without plugin support with a gcc built with plugin support.  Quite
obviously such a combination won't work if using LTO, but gcc has a
habit of passing a bunch of -plugin and -plugin-opt arguments to ld
even when not generating or linking LTO objects.

Why do I want to do this?  Well, it lets me and other binutils
developers answer the question as to whether plugin support breaks
binutils in some areas.  Which it does for some targets.

	* lexsup.c (ld_options): Accept -plugin and -plugin-opt when
	!ENABLE_PLUGINS.
	* testsuite/lib/ld-lib.exp (check_plugin_api_available): Adjust.