[binutils-gdb] [AArch64] Factor out common behaviour between elf_aarch64_create_or_find_stub_sec and elfNN_aarch64_add_stub

[sergiodj+buildbot@redhat.com]

*** TEST RESULTS FOR COMMIT ef8575212574f5cca4426194d7428f900a573852 ***

Author: Marcus Shawcroft <marcus.shawcroft@arm.com>
Branch: master
Commit: ef8575212574f5cca4426194d7428f900a573852

[AArch64] Factor out common behaviour between elf_aarch64_create_or_find_stub_sec and elfNN_aarch64_add_stub