Fix segfault in relocate_erratum_stub on aarch64.

Fix segfault in relocate_erratum_stub on aarch64.

The fix for PR21868 (an internal error when --fix-cortex-a53-843419
is applied) has a small mistake in it. When the stub_owner section
needs an erratum fix an incorrect address for the stubs for the section
is given to relocate_erratum_stub. If we are lucky we will get a segfault;
if we aren't, an incorrect patch or data corruption is possible.
The error is visible in PR21868, but the side-effects aren't fatal.

gold/
	PR gold/22233
	* aarch64.cc (AArch64_relobj::fix_errata_and_relocate_erratum_stubs):
	Fix calculation of stub address.