Missing cgroups support

[rofafor]

The current CPU count detector in Linux only uses affinity:

helio/util/fibers/proactor_pool.cc

Lines 39 to 44 in ceaa6f8

	static cpu_set_t OnlineCpus() {
	cpu_set_t online_cpus;
	CPU_ZERO(&online_cpus);
	CHECK_EQ(0, sched_getaffinity(0, sizeof(online_cpus), &online_cpus));
	return online_cpus;
	}

Modern containerized systems require use of cgroups that is used to limit resource usage and therefore it should be taken account also in Helio's CPU detection. Please, take a look at e.g. OpenJDK's implementation: https://github.com/openjdk/jdk/blob/37c2279148fa91627137e3af362bfcffde61acd0/src/hotspot/os/linux/cgroupSubsystem_linux.cpp#L449-L513

[romange]

wanna send a PR?