Hi all,

I noticed that we have a readings.html page that has pointers to documentation of various backends that GCC supports.
The info on arm seems a bit out of date and somewhat confusing, and there is no entry for aarch64.
This patch tries to address that.

The arm entry is updated to not mention armv2(?) and thumb and an aarch64 entry is added with
a link to the ARM documentation.

Ok to commit?

Thanks,
Kyrill