qemux86_64: Use the core2-64 tune

qemux86_64: Use the core2-64 tune

As x86_64 has been "demoted" to an ABI definition rather than a concrete
tune file, replace it with core2-64 for the qemux86-64 machine.

(From OE-Core rev: 65c1ba225a410d2ee1913d55c6f986db9f54cc8e)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Paul Eggleton <paul.eggleton@intel.com>
Cc: Tom Zanussi <tom.zanussi@intel.com>
Cc: Nitin Kamble <nitin.a.kamble@intel.com>
Cc: Mark Hatle <mark.hatle@windriver.com>
Cc: Bruce Ashfield <bruce.ashfield@windriver.com>
Cc: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>