From Bertrand: fixes for cygwin.
* The PE magic is not within the first 64 bytes (only the DOS "MK" is); need to read more to identify DLLs.
* Use Windows's EnumProcessModules() even on cygwin to enumerate the loaded libraries; needs a Windows library (psapi) to be linked in.
* Filter system libraries on Windows: ignore it if contains "/usr/bin/cyg" or the Windows install path.
* Don't export any symbols from clang and llvm, we don't need them outside libCore. Before, all symbols were exported leading to a symbol table overflow.


git-svn-id: https://root.cern.ch/svn/root/trunk@48500 27541ba8-7e3a-0410-8455-c3a389f83636
Add comment explaining read size of 512 bytes for file magic determination.


git-svn-id: https://root.cern.ch/svn/root/trunk@48501 27541ba8-7e3a-0410-8455-c3a389f83636