[AArch64] Refactor generation of 835769 workaround stubs.

[AArch64] Refactor generation of 835769 workaround stubs.

This patch recognizes that we only need to perform one scan for the
835769 errata and that this scan can take place before we insert
branch stubs.  The erratum scan code is relocated and adjusted to
create stub entries directly rather than populating an intermediate
representation.  Since stub entries are created immediately we can
drop the adhoc stub size adjustment code and allow the generic stub
sizing code to deal with 835769 stub entries.

This patch restructures the code but does not change the workaround
used to deal with erratum 83679, the exact placement of workaround
stubs in the final image may change slightly after this patch due to
stubs being created in a different order.