[x86] Make the 'x86-64' cpu, what I see as and many use as the generic

default architecture for reasonable modern x86 processors, actually be
modern. This processor model should essentially be "tuned" for modern
x86 chips as much as possible without undue penalties on any specific
architecture. Previously we weren't even using the nice scheduling
models. There are a few other tweaks needed here, but this change at
least I have benchmarked across a decent swatch of chips (intel's
clovertown, westmere, and sandybridge; amd's istanbul) and seen no
significant regressions.

If anyone has suggested ways to test this, just let me know. Somewhat
alarmingly, no existing tests failed.

llvm-svn: 208230