Don't set LLVM_NO_DEAD_STRIP on AIX

Summary:
when building plugins, as AIX has symbols in it's standard library that
must be garbage collected or we will see link errors. Export lists will
handle this instead on AIX.

Reviewers: stevewan, sfertile, jasonliu, xingxue, DiggerLin

Reviewed By: DiggerLin

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70130