arm: make linux.h safe to include for non-native builds

arm: make linux.h safe to include for non-native builds

<grub/machine/loader.h> (for machine arm/efi) and
<grub/machine/kernel.h> (for machine arm/coreboot) will not always
resolve (and will likely not be valid to) if pulled in when building
non-native commands, such as host tools or the "file" command.
So explicitly include them with their expanded pathnames.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>