[JITLink] Adds support for PLT based relocations to the ELF/i386 JITLink backend

This commit adds support for PLT based relocations. Specifically -
1. It adds logic to create a `PLTTableManager` in the `buildTables_ELF_i386`
function, which is called as part of the post-prune JITLink passes. The `PLTTableManager`
handles creating pointer jump stubs and related GOT entries for position independent
code.

2. It also adds a pre-fixup pass to optimize away PLT based calls in position independent
code, when possible.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D142846