The google-benchmark library is disabled on Darwin, so we cannot create
host benchmarks. Instead of having every user specify this, put the
logic in Soong. Then if we decide to support it later, it's an easier
switch.
Test: build.ninja identical before/after on Linux
Test: Ignores failing cc_benchmark_host on Darwin
Change-Id: I61f3a571fd160d8e479a512992bc68601f1c9b28