LibCore: Implement System::hardware_concurrency on Windows

Libraries/LibCore/SystemWindows.cpp

@@ -231,4 +231,12 @@ ErrorOr<void> sleep_ms(u32 milliseconds)
     return {};
 }
 
+unsigned hardware_concurrency()
+{
+    SYSTEM_INFO si = {};
+    GetSystemInfo(&si);
+    // number of logical processors in the current group (max 64)
+    return si.dwNumberOfProcessors;
+}
+
 }