What to do about alignment of ELF objects

[Rafael Espíndola]

Currently the lower level APIs keep track of the alignment of elf
objects in memory.

At some point along the layers we switch to just using 2 byte
alignment because of archives.

One option is to just use 2 everywhere.

Another is to copy the data to make it aligned. These patches do just that.

I tested them by linking an X86_64 clang in an ppc64le machine. Having
aligned access seems to help on ppc64le:

trunk:

 2.165775601 seconds time elapsed
    ( +-  2.67% )

patch:

1.979675092 seconds time elapsed
   ( +-  2.96% )

The main missing issue is the memory leak. The reason we can't have
Archive just own the buffer for the unaligned members is that in lld
we have

----------------------------------------
FileVector loadFile(LinkingContext &ctx, StringRef path, bool wholeArchive) {
  ErrorOr<std::unique_ptr<MemoryBuffer>> mb
      = MemoryBuffer::getFileOrSTDIN(path);
  if (std::error_code ec = mb.getError())
    return makeErrorFile(path, ec);
  std::vector<std::unique_ptr<File>> files;
  if (std::error_code ec = ctx.registry().loadFile(std::move(mb.get()), files))
    return makeErrorFile(path, ec);
  if (wholeArchive)
    return parseMemberFiles(files);
  return files;
}
--------------------------------------

So if --whole-archive is used, the Archive itself is freed and we then
try to use the members.

Should the registry have a place for putting Archives?

Cheers,
Rafael
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