Allow users to set the number of build processes (dotnet/coreclr#12188)

Allow users to set the number of build processes (dotnet/coreclr#12188)

* Allow users to set the number of build processes

* Add a command-line option

* Use common initialization style (use __NumProc and extract initialization from build_native)

* Set __NumProc before argument parsing (and replace NumProc with __NumProc)

Commit migrated from https://github.com/dotnet/coreclr/commit/a2235dcc5711bde70f16a846ce88c622d18ae63d