[PATCH] [libcxxabi] Refactor test timing logic and disable by default.

[Jonathan Roelofs]

These are still useful to make sure we don't regress too badly on typeinfo lookup performance, so I think we should default this to 'on'.

Then we can turn it off in the configurations where it makes for unnecessary failures (like on my baremetal stuff where there isn't a monotonic clock).

 It is a bit lame that we don't have a way to automatically catch changes in the perf numbers that come out of this though. Any ideas on how to fix that?

http://reviews.llvm.org/D6391