Add -disable-builtin option to opt

Summary:
The option allows to disable specific target library builtin functions,
instead of -disable-simplify-libcalls, which disables all of them.

This is a prerequisite for D70143, which fixes PR43081.

Reviewers: xbolva00, spatel, jdoerfert, efriedma

Reviewed By: efriedma

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70193