[GlobalISel] add helper function to create arbitrary libcalls

Summary:
The existing helper function can only create a libcall to functions available in
RTLIB. Add a helper function that can create a libcall to a given function name
using the provided calling convention.

Reviewers: aditya_nandakumar, t.p.northover, rovka, arsenm, dsanders

Reviewed By: arsenm

Subscribers: wdng, hiraditya, volkan, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76845