[AIX][lit] Don't depend on psutil on AIX

Summary:
On AIX psutil can run into problems with permissions to read the process
tree, which causes problems for python timeout tests which need to kill off
a test and it's children.

This patch adds a workaround by invoking shell via subprocess and using a
platform specific option to ps to list all the descendant processes so we can
kill them. We add some checks so lit can tell whether timeout tests are
supported with out exposing whether we are utilizing the psutil
implementation or the alternative.

Reviewers: hubert.reinterpretcast, andusy, davide, delcypher

Reviewed By: delcypher

Subscribers: davide, delcypher, christof, lldb-commits, libcxx-commits, llvm-commits

Tags: #lldb, #libc, #llvm

Differential Revision: https://reviews.llvm.org/D64251

llvm-svn: 366912