tests: add squashfs make to m5-exit

tests: add squashfs make to m5-exit

An ARM squashfs rootfs that runs m5 exit can be generated for example
with:

make ARCH=arm_A64 CROSS_COMPILE=aarch64-linux-gnu- squashfs

The existing Makefile.x86 was not used as a basis because we would
like to provide a setup that allows users to use their own compilers
if they wish, without requiring dockcross.

Change-Id: I19c54cf0575b405f191f45aaf1e4a05c3f2e69ae
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22223
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>