elf: Add _dl_audit_pltexit

It consolidates the code required to call la_pltexit audit
callback.

Checked on x86_64-linux-gnu, i686-linux-gnu, and aarch64-linux-gnu.

Reviewed-by: Florian Weimer <fweimer@redhat.com>
(cherry picked from commit 8c0664e2)

Resolved conflicts:
	sysdeps/hppa/dl-runtime.c