[AIX] Make sure we use export lists for plugins

Summary:
Besides just generating and consuming the lists, this includes:

 * Calling  nm with the right options in extract_symbols.py. Such as not
  demangling C++ names, which AIX nm does by default, and accepting both
  32/64-bit names.
 * Not having nm sort the list of symbols or we may run in to memory
   issues on debug builds, as nm calls a 32-bit sort.
 * Defaulting to having LLVM_EXPORT_SYMBOLS_FOR_PLUGINS on for AIX
 * CMake versions prior to 3.16 set the -brtl linker flag globally on
   AIX. Clear it out early on so we don't run into failures. We will set
   it as needed.

Reviewers: jasonliu, DiggerLin, stevewan, hubert.reinterpretcast

Reviewed By: hubert.reinterpretcast

Subscribers: hubert.reinterpretcast, mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70972