elf: Fix tst-pldd for non-default --prefix and/or --bindir (BZ#24544)

elf: Fix tst-pldd for non-default --prefix and/or --bindir (BZ#24544)

Use a new libsupport support_bindir_prefix instead of a hardcoded
/usr/bin to create the pldd path on container directory.

Checked on x86_64-linux-gnu with default and non-default --prefix and
--bindir paths, as well with --enable-hardcoded-path-in-tests.

	[BZ #24544]
	* elf/tst-pldd.c (do_test): Use support_bindir_prefix instead of
	pre-defined value.

Reviewed-by: DJ Delorie <dj@redhat.com>