Add ability for external C code to get pointers to functions given their name.

Add ability for external C code to get pointers to functions given their name.
This us used by bugpoint -- when code is compiled to a shared object to be
JITted, it must use the JIT's lazy resolution method to find function addresses,
because some functions will not be available at .so load time, as they are in
the bytecode file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@7363 91177308-0d34-0410-b5e6-96231b3b80d8