Add a testcase for PR binutils/23460

Add a testcase for PR binutils/23460

Add a testcase to limit open files to 16 for AR with plugin.  Before

commit 103da91bc083f94769e3758175a96d06cef1f8fe
Author: Nick Clifton <nickc@redhat.com>
Date:   Wed Aug 1 14:34:41 2018 +0100

    Close resource leaks in the BFD library's plugin handler.

it failed with:

../binutils/ar: tmpdir/pr23460f.o: plugin needed to handle lto object

	PR binutils/23460
	* testsuite/ld-plugin/lto.exp: Run the PR binutils/23460 test.
	* testsuite/ld-plugin/pr23460a.c: New file.
	* testsuite/ld-plugin/pr23460b.c: Likewise.
	* testsuite/ld-plugin/pr23460c.c: Likewise.
	* testsuite/ld-plugin/pr23460d.c: Likewise.
	* testsuite/ld-plugin/pr23460e.c: Likewise.
	* testsuite/ld-plugin/pr23460f.c: Likewise.