Added 2 configs for Server GC

Added 2 configs for Server GC

complus var GCNoAffinitize or project.json System.GC.NoAffinitize - specify 1/true to disable hard affinity of Server GC threads to CPUs
complus var GCHeapCount or project.json System.GC.HeapCount - specify the # of Server GC threads/heaps, must be smaller than the # of logical CPUs the process is allowed to run on, ie, if you don't specifically affinitize your process it means the # of total logical CPUs on the machine; otherwise this is the # of logical CPUs you affinitized your process to.

Commit migrated from https://github.com/dotnet/coreclr/commit/d088712003e8d483872754d6b3c72aa2d4443a93