Allow bootstrapping llvm-tblgen on macOS and Linux

This is needed in order to build various LLVM binutils (e.g. addr2line)
as well as clang/lld/lldb.

Co-authored-by: Jessica Clarke <jrtc27@FreeBSD.org>
Test Plan: Compiles on ubuntu 18.04 and macOS 11.4
Reviewed By: dim
Differential Revision: https://reviews.freebsd.org/D31057

(cherry picked from commit 31ba4ce8898f9dfa5e7f054fdbc26e50a599a6e3)