Updates and adds tests for i386/ELF JITLink backend

This CR modifies the existing 32 bit pcrel relocation test to
include the case when the relocation target might be present at
a smaller address than the address of the location that needs to be
patched.

Additionally, it adds a test for 16 bit absolute relocation.

Reviewed By: sunho

Differential Revision: https://reviews.llvm.org/D138372