[lld] r249752 - Revert: r249728 - Roll back r249726 and r249723 because they broke buildbots.

[Rafael Espíndola via llvm-commits]

> I had originally thought that they had been causing problems, but going back and restoring the original value (but keeping the page-size fix, which definitely is necessary), sill produces a working hello-world binary. Thus, I don't have a good answer for you, except to say that these are the same magic values that ld.gold uses. That having been said, I'd prefer to keep them this way. Poking around in the debugger, this makes the addresses look more like what I'm used to seeing, and FWIW, matching the behavior of existing linkers here seems like a prudent approach.

So, this is a small example of "cargo coding" I would like to avoid.

Changing the values only when we actually know why their are needed
seems a much better solution. In the X86_64 case that is how we found
out what the restrictions actually were and have them documented in a
comment.

I can see why it has to be 64k aligned on ppc, but the original values was too.

Would you mind reverting the getVAStart part until we actually know of
a reason why a given architecture has to be different?

Thanks,
Rafael