[ORC-RT][ORC] Introduce ELF/*nix Platform and runtime support.

This change adds support to ORCv2 and the Orc runtime library for static
initializers, C++ static destructors, and exception handler registration for
ELF-based platforms, at present Linux and FreeBSD on x86_64. It is based on the
MachO platform and runtime support introduced in bb5f97e3.

Patch by Peter Housel. Thanks very much Peter!

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D108081