Threadpool does not respect process affinity mask

Hi,

Threadpool size defaults to the number of CPU cores (https://github.com/MariaDB/server/blob/e7558d4760d20f7b5480ee2eb3ab7d710d1784b1/sql/mysqld.cc#L3995). However it is possible to bind mysqld to specific CPUs or specific NUMA nodes using numactl, taskset or cgroups cpusets. In this case the number of CPUs available to mysqld is not equal to the number of CPUs available to the OS. It would be nice if threadpool checked affinity mask and adjusted default threadpool size accordingly.