Fix a thinko in the JIT where the address of a GV was only recorded in the map

Fix a thinko in the JIT where the address of a GV was only recorded in the map
  on failure to resolve it.
Do not abort on failure to resolve an external symbol when using dlsym stubs,
  since the symbol may not be in the JIT's address space.  Just use 0.
Allow dlsym stubs to differentiate between GlobalVars and Functions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66050 91177308-0d34-0410-b5e6-96231b3b80d8