OvmfPkg IA32: add support for loading X64 images

This is the UEFI counterpart to my Linux series which generalizes
mixed mode support into a feature that requires very little internal
knowledge about the architecture specifics of booting Linux on the
part of the bootloader or firmware.

Instead, we add a .compat PE/COFF header containing an array of
PE_COMPAT nodes containing <machine type, entrypoint> tuples that
describe alternate entrypoints into the image for different native
machine types, e.g., IA-32 in a 64-bit image so it can be booted
from IA-32 firmware.

This patch implements the PE/COFF emulator protocol to take this new
section into account, so that such images can simply be loaded via
LoadImage/StartImage, e.g., straight from the shell.

This feature is based on the EDK2 specific PE/COFF emulator protocol
that was introduced in commit 57df17fe26 ("MdeModulePkg/DxeCore:
invoke the emulator protocol for foreign images", 2019-04-14).

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2564
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>

OvmfPkg/CompatImageLoaderDxe/CompatImageLoaderDxe.inf

@@ -0,0 +1,37 @@
+## @file
+#  PE/COFF emulator protocol implementation to start Linux kernel
+#  images from non-native firmware
+#
+#  Copyright (c) 2020, ARM Ltd. All rights reserved.<BR>
+#
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
+[Defines]
+  INF_VERSION                    = 1.27
+  BASE_NAME                      = CompatImageLoaderDxe
+  FILE_GUID                      = 1019f54a-2560-41b2-87b0-6750b98f3eff
+  MODULE_TYPE                    = DXE_DRIVER
+  VERSION_STRING                 = 1.0
+  ENTRY_POINT                    = CompatImageLoaderDxeEntryPoint
+
+[Sources]
+  CompatImageLoaderDxe.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+  MdeModulePkg/MdeModulePkg.dec
+
+[LibraryClasses]
+  BaseMemoryLib
+  DebugLib
+  PeCoffLib
+  UefiBootServicesTableLib
+  UefiDriverEntryPoint
+
+[Protocols]
+  gEdkiiPeCoffImageEmulatorProtocolGuid   ## PRODUCES
+
+[Depex]
+  TRUE