elf: Wire up PR ld/21086 test

commit 3c5fce9bc29b216af7d10f8d6e4d8c3f11a48359
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Fri Apr 21 12:00:55 2017 -0700

    Require --no-dynamic-linker with -static -E/--dynamic-list

added pr21086.c and pr21086.list without wiring them up.  Wire them up to
verify that there is no dynamic section with -static -E/--dynamic-list.

	PR ld/21086
	PR ld/33630
	* testsuite/ld-elf/pr21086.rd: New file.
	* testsuite/ld-elf/shared.exp: Wire PR ld/21086 test.

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