[LLVM-C] Add Bindings to GlobalIFunc

Summary:
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

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56177

llvm-svn: 353193