Building RTEMS toolchain on Aarch64 fails on expat

[Alan Cudmore]

Hi,
I tried to use the RSB master branch to build an RTEMS 6 toolchain on
a Raspberry Pi 4 running Ubuntu 20.04 64 bit (Aarch64).
It fails on expat-2.1.0, with the log saying that expat does not
recognize Aarch64.
The expat package on the Ubuntu distribution is 2.2.10.
Would this be as simple as upgrading the expat version in RSB?

I know that building a toolchain on a Raspberry Pi is not the most
common use case, but with the M1 Macs I think this will start to
become a more widely used development platform.

Thanks,
Alan