Add Arch Linux dependency package for testing

[XYenChi]

When using make report command at Arch Linux, need to install the extra python-sphinx python-sphinx_rtd_theme ninja package.

[cmuellner]

ninja is in the list for Ubuntu as well (and in the CI apt-install script), so this is likely missing.

I think that sphinx is an optional dependency for building docs.
Can you try if we can build without sphinx?

[XYenChi]

Yes, building qemu docs need sphinx. And if add the --disable-docs at Makefile.in stamps/build-qemu, can build without sphinx to test gcc.
By the way, building spike with ArchLinux need the device tree compiler dependency, that ArchLinux package name is dtc.