Add some Mac OS X benchmarks

Previously added a benchmark function, but now this just adds some benchmark data from an OS X environment.

<del>This benchmark tests the performance of the runtime when opening thousands of goroutines simultaneously, and leaving them open. It may effectively be testing the internal allocation routines, but there seems to be a [substantial performance regression](https://github.com/nickstenning/autobench/blob/965fba8f965b7a26d0d50bb672e996c5866e8b32/darwin-amd64-runtime-112ea225794f.txt#L79) in 1.1.</del>