Implement GetCPUCount for Windows OSes. This is for all three Windows targets.

rtl/win/sysos.inc:
  + add definition of TSystemInfo structure
  + add import of GetSystemInfo
  + add implementation of GetCPUCount which returns value for number of processors retrieved by GetSystemInfo


git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@28643 3ad0048d-3df7-0310-abae-a5850022a9f2