minimum gcc version for Arm64

[Jan Beulich <jbeulich@xxxxxxxx>]

Hello,

by way of monitoring the stable Linux kernel logs I've become
aware of Linux commit dca5244d2f5b raising the minimum version
to 5.1. For Arm we currently document 4.8 - should this be
adjusted (and perhaps split for Arm32 and Arm64)?

Jan