Default ISA/tuning flags for GCC, --enable-kernel= level for glibc

[Kevin Kofler]

I wrote:
> But Koji just uses a plain chroot, not actual virtualization at all.

To be more precise, some Koji build hosts are running Xen, but the Xen
guests are also EL5 guests, it uses mock chroots inside those.

        Kevin Kofler