[binutils-gdb] [AArch64] Use _bfd_aarch64_add_stub_entry_in_group.

[sergiodj+buildbot@redhat.com]

*** TEST RESULTS FOR COMMIT e572930b96d8080ec80e65cfe85aaa8c4f7b2603 ***

Author: Marcus Shawcroft <marcus.shawcroft@arm.com>
Branch: master
Commit: e572930b96d8080ec80e65cfe85aaa8c4f7b2603

[AArch64] Use _bfd_aarch64_add_stub_entry_in_group.
Adjust the 835769 workaround code to use
_bfd_aarch64_add_stub_entry_in_group rather than inspect the
underlying section_group structure directly.