Set affinity to none/false in child processes

Reset affinity to none (false for proc-bind-var) so that threads in the child
processes are not bound tightly, unless the user explicitly sets this in
KMP_AFFINITY/OMP_PROC_BIND, in child processes. This can improve
performance for scripting languages which fork for parallelism like Python's
multiprocessing module.

Differential Revision: https://reviews.llvm.org/D34154

llvm-svn: 305513