Thoroughly rehack the dynamic linking mechanisms on Win32. The Win32

Thoroughly rehack the dynamic linking mechanisms on Win32.  The Win32
dynamic linker does not automatically search libraries when looking up
symbols with GetProcAddress.  Because of this we have to emulate it.  The
only detail is that there doesn't seem to be a way to enumerate the
libraries loaded, so we have a gross hack (tm).

This make the JIT functional on win32 under cygwin.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13887 91177308-0d34-0410-b5e6-96231b3b80d8