performance_fast_pow is failing on OSX x86 buildbot

[steven-johnson]

This has been happening reliably for about a week or so, but:

• it doesn't seem to happen on any other Mac or x86 machine, either on the buildbots or locally
• it only seems to happen when this test is run as part of the buildbot process; logging into the buildbot and running the same executable doesn't fail

It's extremely repeatable on the buildbot, so something is going on, but it's not at all clear what that is.

[steven-johnson]

e.g., from buildbot logs:

 7/30 Test #512: performance_fast_pow ....................***Failed  Required regular expression not found. Regex=[Success!
]  1.55 sec
powf: 31.403805 ns per pixel
Halide's pow: 3.761951 ns per pixel (rms error = 0.0000001623)
Halide's fast_pow: 6.674909 ns per pixel (rms error = 0.0000019084)
pow is more than 1.5x faster than fast_pow

from running locally on the same machine:

powf: 32.184489 ns per pixel
Halide's pow: 3.954797 ns per pixel (rms error = 0.0000001623)
Halide's fast_pow: 2.035149 ns per pixel (rms error = 0.0000019084)
Success!