qemux86-64: Reduce tuning to core2-64

Effectively revert "qemux86-64: build for x86-64-v3 (2013 Haswell and later) rather than Core 2 from 2006"
(commit 6f2af1e5)

Much as I'd love us to use the latest tuning, we do have some autobuilder
hardware which isn't ready for this yet which breaks KVM and some qemu
user mode usage as there appear to be TCG bugs too. I suspect we're not
the only ones with such hardware.

Drop the tune back to core2-64, anyone can easily customise it
themselves if they need it. We can revisit this in a year or two
as we should be ready then. It has beena  good test of the rest of the
support which all seems ready.

I'd have preferred to use corei7-64 but that causes
runqemu.RunqemuTests.test_boot_machine_iso to hang.

Leave the newer tune file inclusion so people can change tunes
more easily.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>