efi_loader: Use compiler constants for image loader

The EFI image loader tries to determine which target architecture we're
working with to only load PE binaries that match.

So far this has worked based on CONFIG defines, because the target CPU
was always indicated by a config define. With sandbox however, this is
not longer true as all sandbox targets only encompass a single CONFIG
option and so we need to use compiler defines to determine the CPU
architecture.

Signed-off-by: Alexander Graf <agraf@suse.de>