ext: Add an Arm section for gem5-SST README.md Change-Id: Ia10948a836797855219aef406bf38f52d28d8e26 Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com> Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/53628 Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu> Tested-by: kokoro <noreply+kokoro@google.com>