[LLVM-C] Add Bindings to GlobalIFunc
Adds the standard gauntlet of accessors for global indirect functions and updates the echo test.
Now it would be nice to have a target abstraction so one could know if they have access to a suitable ELF linker and runtime.
Reviewers: whitequark, deadalnix
Reviewed By: whitequark
Differential Revision: https://reviews.llvm.org/D56177