plugin: Copy the no_export field to the IR dummy object

Copy the no_export field to the IR dummy object when claiming an IR
object.

	PR ld/25618
	* plugin.c (plugin_object_p): Copy the no_export field to the IR
	dummy object.
	* testsuite/ld-plugin/lto.exp (lto_link_elf_tests): Add
	PR ld/25618 tests.
	* testsuite/ld-plugin/pr25618.d: New file.
	* testsuite/ld-plugin/pr25618a.cc: Likewise.
	* testsuite/ld-plugin/pr25618a.h: Likewise.
	* testsuite/ld-plugin/pr25618b.cc: Likewise.
	* testsuite/ld-plugin/pr25618b.h: Likewise.