[patch][gold] Use LIFO instead of FIFO to implement gc's transitive closure

[Rafael Espíndola]

> I passed this by Sri (the original author of the --gc-sections code)
> to see if he had any objections, and he's fine with it.

Thanks. Just pushed it. Sorry for the delay. I was busy with the llvm
dev meeting (and company meetings).

> This is OK, with a ChangeLog entry. Please try to remember to include
> ChangeLog entries with your patches (but not as *part* of the patches
> -- including it with the plain text of the message helps the reviewer,
> whereas including it in the patch makes merges messy).

I had never thought of ChangeLogs as useful for review (or at all :-)
), but if you find them useful I will try to include them for the next
patches.

Cheers,
Rafael