SystemInformation: Expose helper functions only where needed

Drop unused helper GetFieldFromCommand.  Define helpers

 LoadLines
 NameValue
 GetFieldFromFile
 GetFieldsFromFile
 GetFieldsFromCommand
 StacktraceSignalHandler

only on platforms where they are used.  Hide all helpers in an anonymous
namespace so they are not visible outside the SystemInformation
translation unit.

Change-Id: I514881a8f253d72e90a0fb8bf1a6d7d9bc7e01c5