[gold] Merging string literals with bigger alignment

[Cary Coutant]

> There was no feedback from Ian for this patch: I rebased it and did
> the changes that you mentioned.
> As a "fully fledged"(c) gold maintainer (congratulations btw :) )
> could you please take a look again?
>
> All tests pass on x86-64-gnu-linux.
> If ok, could someone commit please? I don't have commit access.

Committed on your behalf. Thanks!

-cary