[patch][gold] Avoid invalid unaligned write

[Cary Coutant]

> 2015-06-07  Rafael Ávila de Espíndola  <rafael.espindola@gmail.com>
>
> * elfcpp_swap.h (Swap::writeval): Avoid unaligned access.

We already have Swap and Swap_unaligned templates. Rather than
pessimize the Swap template, we should be using Swap_unaligned
wherever the target of the relocation might be unaligned. The arm,
powerpc, and sparc targets already do this. The i386 and x86_64
targets don't, but we haven't got into trouble only because
(presumably) no one has ever tried to build a cross-linker for those
targets on a platform where aligned accesses are required.

I'd prefer to see _unaligned variants of all necessary functions in
Relocate_functions, and have i386 and x86_64 targets use those. The
mips target may also need this.

Before doing that, however, the non-byte-swapping versions of the
Swap_unaligned class templates need to be rewritten to use memcpy so
that we don't generate horrible code when hosted on a platform that
supports unaligned access. I'm not sure yet whether the byte-swapping
versions should remain as written (byte-at-a-time, taking care of
unaligned loads and byte swapping together), or whether they should
use memcpy and Convert::convert_host. (The latter would be far
simpler.)

As you've pointed out, unaligned accesses on x86 aren't hurting us, so
I don't see this as a high priority. It's just something to take care
of before someone tries to build a cross linker for x86 hosted on a
platform that doesn't tolerate unaligned access.

-cary