Fix build parallelism on Windows with NUMA (#21278)

Fix build parallelism on Windows with NUMA (#21278)

When multiple NUMA nodes are enabled on the machine where coreclr is
built, we incorrectly detect the number of cores that we use for build
parallelism of the native part of the build (NumberOfCores) as only
a number in the last NUMA node.
The reason is that the `wmic cpu get NumberOfCores /value` returns
multiple lines, one per each NUMA node.
This change fixes it by summing values from all the lines.