[PATCH][gold, aarch64] Skip ERRATUM 843419 fix if the sequences have been relaxed by TLS optimization

[Cary Coutant]

> Yup, let's change "return  false;" -> "return true;" (and add proper comments).
> Also, I assume this relaxation-changing-instructions behavior should
> not break erratum 835769? Erratum 843419 must have adrp as its first
> insn, whereas 835769 begins with LD/ST.
>
> Otherwise, LGTM

This is OK with me (with Han's suggested changes). It doesn't look
like this is related to any outstanding PR, but if it is, please add
the PR number to the ChangeLog.

-cary