x86/efi: Add early thunk code to go from 64-bit to 32-bit

Implement the transition code to go from IA32e mode to protected mode in
the EFI boot stub. This is required to use 32-bit EFI services from a
64-bit kernel.

Since EFI boot stub is executed in an identity-mapped region, there's
not much we need to do before invoking the 32-bit EFI boot services.
However, we do reload the firmware's global descriptor table
(efi32_boot_gdt) in case things like timer events are still running in
the firmware.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>

diff --cc arch/x86/boot/compressed/efi_stub_64.S
Simple merge

diff --cc arch/x86/platform/efi/efi_stub_64.S
Simple merge