Replace uses of GRUB_LINUX_MACHINE_HEADER and GRUB_LINUX_CPU_HEADER
with GRUB_<arch>_LINUX_HEADER include guards to prevent issues when
including more than one of them.
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>