ld/testsuite: Use $plug_opt for --plugin option

Use $plug_opt for --plugin usage, instead of running:

run_host_cmd "$CC_FOR_TARGET" "-print-prog-name=liblto_plugin.so"

	PR binutils/21479
	* testsuite/ld-plugin/lto-binutils.exp (lto_plugin): Removed.
	Replace "--plugin $lto_plugin" with $plug_opt.
	* testsuite/ld-plugin/lto.exp (lto_plugin): Removed.
	Replace "--plugin $lto_plugin" with $plug_opt.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>