Actually refer to the base relocation table of our loaded image.

Currently when we process base relocations, we get the correct Data
Directory pointer from the headers (context->RelocDir), and that header
has been copied into our pristine allocated image when we copied up to
SizeOfHeaders.  But the data it points to has not been mirrored in to
the new image, so it is whatever data AllocPool() gave us.

This patch changes relocate_coff() to refer to the base relocation table
from the image we loaded from disk, but apply the fixups to the new
copy.

I have no idea how x86_64 worked without this, but I can't make aarch64
work without it.  I also don't know how Ard or Leif have seen aarch64
work.  Maybe they haven't?  Leif indicated on irc that they may have
only tested shim with simple "hello world" applications from gnu-efi;
they are certainly much less complex than grub.efi, and are generated
through a different linking process.

My only theory is that we're getting recycled data there pretty reliably
that just makes us /not/ process any relocations, but since our
ImageBase is 0, and I don't think we ever load grub with 0 as its base
virtual address, that doesn't follow.  I'm open to any other ideas
anybody has.

I do know that on x86_64 (and presumably aarch64 as well), we don't
actually start seeing *symptoms* of this bug until the first chunk[0] of
94c9a77f is applied[1].  Once that is applied, relocate_coff() starts
seeing zero[2] for both RelocBase->VirtualAddress and
RelocBase->SizeOfBlock, because RelocBase is a (generated, relative)
pointer that only makes sense in the context of the original binary, not
our partial copy.  Since RelocBase->SizeOfBlock is tested first,
relocate_base() gives us "Reloc block size is invalid"[3] and returns
EFI_UNSUPPORTED.  At that point shim exits with an error.

[0] The second chunk of 94c9a77f patch makes no difference on this
    issue.
[1] I don't see why at all.
[2] Which could really be any value since it's AllocatePool() and not
    AllocateZeroPool() results, but 0 is all I've observed; I think
    AllocatePool() has simply never recycled any memory in my test
    cases.
[3] which is silent because perror() tries to avoid talking because that
    has caused much crashing in the past; work needs to go in to 0.9 for
    this.

Signed-off-by: Peter Jones <pjones@redhat.com>