In FreeBSD when run Go proc under a given sub-list of
processors(e.g. 'cpuset -l 0 ./a.out' in multi-core system),
runtime.NumCPU() still return all physical CPUs from sysctl
hw.ncpu instead of account from sub-list.

Fix by use syscall cpuset_getaffinity to account the number of sub-list.

Fixes #15206

Change-Id: If87c4b620e870486efa100685db5debbf1210a5b
Reviewed-on: https://go-review.googlesource.com/29341Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>