Bug 24260 – GC creates too many threads when running with restricted CPU affinity

Currently, the GC decides on the number of threads to create (for parallel marking) by querying the information in CPUID. This is not ideal, because a process may be created with a CPU/core affinity restricting which CPUs/cores it may use, e.g. using the "taskset" utility. In this case, it will create too many threads. It should instead use the information provided from the OS, in the same way that std.parallelism.totalCPUs does. (totalCPUs should probably be moved to Druntime.)

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