Run ar with --plugin for LTO 11 test

Run ar with --plugin for LTO 11 test

Objects built with -fno-fat-lto-objects (the default for gcc-4.9) have
no normal symbols, except things like __gnu_lto_slim.  These useless
symbols are the ones put into the archive index, and of course nothing
references them so no objects are extracted by the linker.  Running
ar with --plugin changes ar behaviour to put the lto symbols into the
archive index.

	PR 17112
	* ld-plugin/lto.exp: When building liblti-11.a, pass
	--plugin path_to_gcc/liblto_plugin.so to ar.