ld: Restore PR ld/15146 tests

commit 7d409ac0
Author: Alan Modra <amodra@gmail.com>
Date:   Thu Feb 4 13:56:34 2021 +1030

    PR27311, (symbol from plugin): undefined reference, hidden sym

fixed PR ld/15146.  Restore PR ld/15146 tests.

	PR ld/15146
	* testsuite/ld-plugin/lto.exp: Run PR ld/15146 tests.
	* testsuite/ld-plugin/pr15146.d: Restored.
	* testsuite/ld-plugin/pr15146a.c: Likewise.
	* testsuite/ld-plugin/pr15146b.c: Likewise.
	* testsuite/ld-plugin/pr15146c.c: Likewise.
	* testsuite/ld-plugin/pr15146d.c: Likewise.

(cherry picked from commit 37707bd8)