perf_counters: Initialize once only when needed

[jmr]

This works around some performance problems running Android under QEMU. Calling pfm_initialize was very slow, and was called during dynamic initialization (before main or when loaded as a shared library). This happened whenever benchmark was linked, even if no benchmarks were run.

Instead, call pfm_initialize at most once, and only when one of:

1. PerfCounters::Initialize is called
2. PerfCounters::Create is called with a non-empty counter list
3. PerfCounters::IsCounterSupported is called

The return value of the first pfm_initialize() is saved and returned from all subsequent PerfCounters::Initialize calls.

[jmr]

@mtrofin

[dmah42]

thanks!