efi_loader: move HOST_ARCH to version_autogenerated.h

efi_default_filename.h requires HOST_ARCH to be defined. Up to now we
defined it via a CFLAGS. This does not scale. Add the symbol to
version_autogenerated.h instead.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>